feat(py): step 1 refinement — error-path example configs for simple.py

- python/configs/: 7 purpose-built config files, one per error class:
  all-ok.txt, dns-failure.txt, connection-refused.txt, timeout.txt,
  tls-errors.txt (badssl.com), http-errors.txt (real 404 paths on stable
  hosts), mixed.txt (one of each, no timeout)
- python/configs/usage.md: runnable sh commands + expected output per config,
  plus quick-sweep and full-sweep loops; commands written for running from
  the python/ directory
- docs/custom-usage-examples.md: example 11 added (all failure classes at once)
- docs/usage/py-simple.md: added pointer to python/configs/ and corrected
  limitation note (4xx/5xx are FAIL, not OK)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/custom-usage-examples.md

@@ -284,6 +284,40 @@ JSON output lets you pipe results to `jq` for filtering.
 
 ---
 
+## Example 11 — 3 working sites, one of each error kind (exit code 6)
+
+Minimal URL set that exercises every failure class simultaneously.
+Three real sites succeed; five targets each trigger a distinct error.
+`--fail` is required to surface the HTTP 4xx as an exit code.
+**Expected exit code: 6** (highest code wins; HTTP error = 6 > TLS = 5 > timeout = 4 > refused = 3 > DNS = 2).
+
+```sh
+./go/latprobe -n 5 --fail --timeout 3s \
+  https://www.cloudflare.com \
+  https://www.github.com \
+  https://www.wikipedia.org \
+  https://nonexistent-host.invalid \
+  http://127.0.0.1:1 \
+  https://192.0.2.1 \
+  https://self-signed.badssl.com \
+  https://httpbin.org/status/404
+```
+
+| URL | Expected outcome | Exit-code contribution |
+| --- | --------------- | ---------------------- |
+| cloudflare.com | success | — |
+| github.com | success | — |
+| wikipedia.org | success | — |
+| nonexistent-host.invalid | DNS failure | 2 |
+| 127.0.0.1:1 | connection refused | 3 |
+| 192.0.2.1 | timeout (3 s × 5 samples) | 4 |
+| self-signed.badssl.com | TLS handshake failure | 5 |
+| httpbin.org/status/404 | HTTP 404 (with --fail) | 6 |
+
+_Estimated runtime: ~18–22 s — 8 workers cover all URLs in parallel; the non-routable IP (192.0.2.1) drives the wall time at 3 s × 5 samples = 15 s._
+
+---
+
 ## Exit code reference
 
 | Code | Meaning |

python/configs/all-ok.txt

@@ -0,0 +1,7 @@
+# All sites are expected to respond successfully.
+# Run: python3.14 python/simple.py python/configs/all-ok.txt
+# Expected exit code: 0
+
+https://example.com
+https://www.google.com
+https://www.iana.org

python/configs/connection-refused.txt

@@ -0,0 +1,7 @@
+# Connection-refused errors — loopback addresses with no server on those ports.
+# The OS rejects the TCP SYN immediately, so these fail in milliseconds.
+# Run: python3.14 python/simple.py python/configs/connection-refused.txt
+# Expected exit code: 1
+
+http://127.0.0.1:9999
+http://127.0.0.1:19999

python/configs/dns-failure.txt

@@ -0,0 +1,9 @@
+# DNS resolution failures — hostnames that cannot be resolved.
+# .invalid is an IANA-reserved TLD guaranteed never to resolve (RFC 2606).
+# Run: python3.14 python/simple.py python/configs/dns-failure.txt
+# Expected exit code: 1
+
+https://this-host-does-not-exist.invalid
+http://no.such.host.invalid
+
+# Also exercises the blank-line and comment-line parser paths.

python/configs/http-errors.txt

@@ -0,0 +1,21 @@
+# HTTP error status codes — urllib.request.urlopen() raises HTTPError for
+# 4xx/5xx responses, so simple.py reports these as FAIL even though the server
+# responded. The URLs below are real paths that reliably return 404 on stable
+# public servers.
+#
+# Note: a genuine 500 from a well-known server is hard to provoke on demand.
+# These 404s are sufficient to demonstrate that HTTP errors are treated as FAIL.
+#
+# Requires internet access.
+#
+# Run: python3.14 python/simple.py python/configs/http-errors.txt
+# Expected exit code: 1
+
+# 404 from Google
+https://www.google.com/this-page-does-not-exist-at-all-1234567890
+
+# 404 from GitHub
+https://github.com/this-repo-does-not-exist-abcxyz123/no-way
+
+# 404 from IANA
+https://www.iana.org/this-page-does-not-exist-either

python/configs/mixed.txt

@@ -0,0 +1,21 @@
+# Mixed — one entry from each error class alongside a successful site.
+# Shows that OK and FAIL lines can interleave in the same run.
+# Timeout is omitted here so the run completes in a few seconds.
+#
+# Run: python3.14 python/simple.py python/configs/mixed.txt
+# Expected exit code: 1  (any FAIL drives exit to 1)
+
+# Success
+https://example.com
+
+# DNS failure
+https://no.such.host.invalid
+
+# Connection refused (loopback, no server)
+http://127.0.0.1:9999
+
+# TLS certificate error
+https://self-signed.badssl.com/
+
+# HTTP error (server responded with 404)
+https://github.com/this-repo-does-not-exist-abcxyz123/no-way

python/configs/timeout.txt

@@ -0,0 +1,14 @@
+# Timeout — non-routable IP addresses that accept no TCP traffic.
+# The kernel sends a SYN but never gets a reply; simple.py waits the full
+# 10-second hard-coded timeout per host before printing FAIL.
+#
+# WARNING: this config takes ~20 seconds to complete (10 s per host).
+#
+# 10.255.255.1 and 192.0.2.1 (RFC 5737 TEST-NET-1, documentation-only range)
+# are guaranteed to be unreachable on any normal network.
+#
+# Run: python3.14 python/simple.py python/configs/timeout.txt
+# Expected exit code: 1
+
+http://10.255.255.1/
+http://192.0.2.1/

python/configs/tls-errors.txt

@@ -0,0 +1,21 @@
+# TLS certificate errors — badssl.com provides endpoints with intentionally
+# broken certificates. Python's ssl module rejects them by default.
+#
+# Note: badssl.com may intermittently reset the connection instead of
+# completing the TLS handshake. The error message will then read
+# "[Errno 54] Connection reset by peer" rather than CERTIFICATE_VERIFY_FAILED,
+# but simple.py still correctly reports FAIL in either case.
+#
+# Requires internet access.
+#
+# Run: python3.14 python/simple.py python/configs/tls-errors.txt
+# Expected exit code: 1
+
+# Certificate has expired
+https://expired.badssl.com/
+
+# Certificate is self-signed (not trusted by the system CA store)
+https://self-signed.badssl.com/
+
+# Certificate chain is incomplete (intermediate CA missing)
+https://incomplete-chain.badssl.com/

python/configs/usage.md

@@ -0,0 +1,185 @@
+# `simple.py` — Example Config Files
+
+Each `.txt` file in this directory targets a distinct error path so you can
+observe `simple.py`'s behaviour for every failure class. Run them from the
+`python/` directory.
+
+---
+
+## `all-ok.txt` — all sites respond successfully
+
+```sh
+python simple.py configs/all-ok.txt
+echo "exit: $?"
+```
+
+Expected output:
+```
+OK        xxx.xx ms  https://example.com
+OK        xxx.xx ms  https://www.google.com
+OK        xxx.xx ms  https://www.iana.org
+exit: 0
+```
+
+---
+
+## `dns-failure.txt` — DNS resolution fails
+
+Hostnames use the `.invalid` TLD (RFC 2606), which is guaranteed never to
+resolve. Fails in milliseconds.
+
+```sh
+python simple.py configs/dns-failure.txt
+echo "exit: $?"
+```
+
+Expected output:
+```
+FAIL   ([Errno 8] nodename nor servname provided, or not known)  https://this-host-does-not-exist.invalid
+FAIL   ([Errno 8] nodename nor servname provided, or not known)  http://no.such.host.invalid
+exit: 1
+```
+
+---
+
+## `connection-refused.txt` — TCP connection refused
+
+Loopback addresses with no server listening. The OS rejects the SYN immediately
+(sub-millisecond failure).
+
+```sh
+python simple.py configs/connection-refused.txt
+echo "exit: $?"
+```
+
+Expected output:
+```
+FAIL   ([Errno 61] Connection refused)  http://127.0.0.1:9999
+FAIL   ([Errno 61] Connection refused)  http://127.0.0.1:19999
+exit: 1
+```
+
+---
+
+## `timeout.txt` — connect hangs until timeout
+
+Non-routable IPs (private/documentation ranges) silently drop TCP SYNs.
+`simple.py` waits the full **10-second** timeout per host.
+
+> ⚠️ This run takes approximately **20 seconds** to complete.
+
+```sh
+python simple.py configs/timeout.txt
+echo "exit: $?"
+```
+
+Expected output (after ~20 s):
+```
+FAIL   (timed out)  http://10.255.255.1/
+FAIL   (timed out)  http://192.0.2.1/
+exit: 1
+```
+
+---
+
+## `tls-errors.txt` — TLS certificate errors
+
+Uses [badssl.com](https://badssl.com) endpoints with intentionally broken
+certificates. Requires internet access; badssl.com must be reachable.
+
+```sh
+python simple.py configs/tls-errors.txt
+echo "exit: $?"
+```
+
+Expected output:
+```
+FAIL   ([SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: certificate has expired …)     https://expired.badssl.com/
+FAIL   ([SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self-signed certificate …)     https://self-signed.badssl.com/
+FAIL   ([SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer …)  https://incomplete-chain.badssl.com/
+exit: 1
+```
+
+> **Note:** badssl.com occasionally resets connections mid-handshake. When that
+> happens, the error reads `[Errno 54] Connection reset by peer` instead of
+> `CERTIFICATE_VERIFY_FAILED`. The key behaviour is the same — every entry is
+> reported as `FAIL` and the script exits 1. This demonstrates that `simple.py`
+> handles both SSL-level and network-level TLS failures uniformly.
+
+---
+
+## `http-errors.txt` — HTTP 4xx / 5xx status codes
+
+`urllib.request.urlopen()` raises `HTTPError` for 4xx/5xx, so `simple.py`
+reports these as `FAIL` even though the server responded. The URLs are real
+paths that reliably return 404 on stable public servers. Requires internet
+access.
+
+```sh
+python simple.py configs/http-errors.txt
+echo "exit: $?"
+```
+
+Expected output:
+```
+FAIL   (HTTP Error 404: Not Found)  https://www.google.com/this-page-does-not-exist-at-all-1234567890
+FAIL   (HTTP Error 404: Not Found)  https://github.com/this-repo-does-not-exist-abcxyz123/no-way
+FAIL   (HTTP Error 404: Not Found)  https://www.iana.org/this-page-does-not-exist-either
+exit: 1
+```
+
+---
+
+## `mixed.txt` — one of each (no timeout)
+
+One successful site followed by one entry from each fast error class (DNS,
+connection-refused, TLS, HTTP). Timeout is excluded so the run completes
+quickly.
+
+```sh
+python simple.py configs/mixed.txt
+echo "exit: $?"
+```
+
+Expected output:
+```
+OK        xxx.xx ms  https://example.com
+FAIL   ([Errno 8] nodename nor servname …)        https://no.such.host.invalid
+FAIL   ([Errno 61] Connection refused)             http://127.0.0.1:9999
+FAIL   ([SSL: CERTIFICATE_VERIFY_FAILED] …)       https://self-signed.badssl.com/
+FAIL   (HTTP Error 404: Not Found)                 https://github.com/this-repo-does-not-exist-abcxyz123/no-way
+exit: 1
+```
+
+> **Note:** the TLS entry may occasionally show `[Errno 54] Connection reset by peer`
+> instead of `CERTIFICATE_VERIFY_FAILED` — badssl.com sometimes drops the connection
+> before the handshake completes. Both are reported as `FAIL`; run `tls-errors.txt`
+> in isolation for consistent cert-verification messages.
+
+---
+
+## Run all configs
+
+Iterate over every config file (skip `timeout.txt` for a quick sweep, or omit
+the exclusion to run all):
+
+```sh
+# Quick sweep — skips the slow timeout config
+for f in configs/*.txt; do
+    [[ "$f" == *timeout* ]] && continue
+    echo
+    echo "=== $f ==="
+    python simple.py "$f"
+    echo "exit: $?"
+done
+```
+
+```sh
+# Full sweep — includes timeout (~20 s extra)
+for f in configs/*.txt; do
+    echo
+    echo "=== $f ==="
+    python simple.py "$f"
+    echo "exit: $?"
+done
+```