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 |