feat(hxprobe): httpx-based HTTP probe — full standalone package

Third Python implementation in the latency-tool progression.
Mirrors latprobe's feature set but replaces raw sockets with httpx,
gaining HTTP/2 support and redirect following.

Package layout (hxprobe/hxprobe/):
- probe.py: asyncio + httpx.AsyncClient with a _TimingStream transport
  wrapper that captures DNS/connect/TLS/TTFB/transfer phase timings via
  httpx event hooks (get_connection_stats, request_started, etc.).
  VerboseDetail captures resolved IP, TLS version/cipher/bits, cert CN/
  expiry/issuer (from httpx's SSLObject), and response headers.
  Options: timeout, verbose, follow_redirects, http2
- aggregate.py: summarize() → per-phase min/avg/max (same schema as latprobe)
- cli.py: run(args,stdout,stderr)->int; argparse with redirect_stdout/
  redirect_stderr + SystemExit catch; -n/--count, -c/--concurrency,
  --timeout, --fail, --json, -v/--verbose, --no-follow-redirects,
  --no-http2, -f/--file (URL list from file, mutually exclusive with args);
  multi-URL summary footer (tally + exit label) when len(urls) > 1;
  worst-exit-code logic mirrors Go/latprobe; JSON output is bare array
- duration.py: same parse_duration() as latprobe
- Exit codes: 0 ok, 1 usage, 2 dns, 3 connect, 4 timeout, 5 tls, 6 http≥400

Tests (hxprobe/tests/):
- test_probe.py: 18 hermetic tests using anyio + in-process ASGI servers
- test_cli.py: 36 hermetic tests (success, failures, JSON, verbose, -f flag,
  run-summary footer, worst-code accumulation)
- test_integration.py: pytest-marked @integration (excluded from hx-test)

Toolchain: uv + ruff + pytest; pyproject.toml with [dependency-groups];
hxprobe/Makefile standalone (help, deps, run, lint, fmt, test,
test-integration, check, clean); .python-version pins 3.14
Configs: 8 fixture files mirroring python/configs/ (all-ok through mixed)
USAGE.md: 16 runnable examples with real captured output

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hxprobe/configs/http-errors.txt

@@ -0,0 +1,22 @@
+# HTTP error status codes — without --fail, hxprobe treats 4xx/5xx as a
+# normal (successful) probe outcome, matching Go's http.DefaultClient
+# semantics. --fail opts back into "HTTP error = failure", which is what
+# this fixture demonstrates. 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 --fail's effect on the exit code.
+#
+# Requires internet access.
+#
+# Run: uv run python -m hxprobe --fail -f configs/http-errors.txt
+# Expected exit code: 6  (0 without --fail — the requests still succeed)
+
+# 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