feat(py): step 1 — simple reachability checker (simple.py)

- python/simple.py: reads a plain-text site list (one URL/line, # comments),
  issues a GET per site, prints aligned OK/FAIL + elapsed ms, exits 0/1
- Config format forward-compatible with future key=value annotations
- HTTPError caught separately from URLError so HTTP status code appears in
  the FAIL message (e.g. "HTTP Error 404: Not Found")
- python/sites.txt: committed example config
- Makefile: PYTHON/PY_DIR/SITES vars; py-simple-run, py-phases-run, py-run,
  py-test, py-check, py-clean targets; umbrella test/check/clean include Python
- docs/plans/2026-07-01-10-39-py-simple.md: design plan
- docs/usage/py-simple.md: flags, format, examples, limitations

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

docs/usage/py-simple.md

@@ -0,0 +1,79 @@
+# `simple.py` — Site Reachability Checker
+
+## What it does
+
+Reads a plain-text list of URLs, issues an HTTP GET to each one, and reports
+whether the site responded and how long it took (full wall-clock time including
+body download). This is the simplest possible latency check — one line of
+output per site, nothing more.
+
+## Flags / arguments
+
+```
+python simple.py [sites.txt]
+```
+
+| Argument | Default | Meaning |
+|----------|---------|---------|
+| `sites.txt` | `sites.txt` in the current directory | Path to the plain-text config file |
+
+**Config file format:**
+- One URL per line.
+- Lines starting with `#` are comments and are ignored.
+- Blank lines are ignored.
+- Future `key=value` tokens after the URL (e.g. `https://x.com  budget=200ms`)
+  are silently ignored, so the file format is forward-compatible.
+
+**Exit codes:**
+
+| Code | Meaning |
+|------|---------|
+| 0 | All sites responded |
+| 1 | One or more sites failed, or a usage/config error |
+
+## Example
+
+**Config (`sites.txt`):**
+```
+https://example.com
+https://www.google.com
+# https://httpbin.org/get   # uncomment to include
+```
+
+**Run:**
+```sh
+python simple.py sites.txt
+```
+
+**Expected output (latencies vary):**
+```
+OK        147.11 ms  https://example.com
+OK         83.42 ms  https://www.google.com
+```
+
+**With an unreachable host:**
+```
+OK        147.11 ms  https://example.com
+FAIL   (nodename nor servname provided, or not known)  https://nonexistent.invalid
+```
+Exit code: `1`
+
+## Example config files
+
+`python/configs/` contains purpose-built config files targeting every distinct
+failure class (DNS, connection-refused, timeout, TLS-cert errors, HTTP 4xx/5xx,
+and a mixed scenario). Each file includes a comment explaining what it exercises.
+
+See [`python/configs/usage.md`](../../python/configs/usage.md) for the runnable
+shell commands and expected output for each file.
+
+## Limitations
+
+- Measures total wall-clock time (DNS + TCP + TLS + server + transfer) as a
+  single number. Use `phases.py` for a per-phase breakdown.
+- Always uses GET; no auth, no custom headers, no redirect control.
+- Timeout is hard-coded at 10 s. Use `phases.py` or `latprobe/` for a `--timeout`
+  flag.
+- HTTP error statuses (4xx, 5xx) are reported as `FAIL` — `urllib.request.urlopen`
+  raises `HTTPError` for non-2xx responses, so a 404 is treated as a failure,
+  not a success.