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/mixed.txt

@@ -0,0 +1,23 @@
+# Mixed — one entry from each error class alongside a successful site.
+# Shows that OK and FAIL lines can interleave in the same run, and that the
+# exit code is the *worst* code across every URL (2=dns, 3=connect, 5=tls,
+# 6=http-with-fail — 6 wins here). Timeout is omitted so the run completes
+# in a few seconds; --fail is needed for the 404 line to count as a failure.
+#
+# Run: uv run python -m hxprobe --fail -f configs/mixed.txt
+# Expected exit code: 6
+
+# 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; needs --fail to count as a failure)
+https://github.com/this-repo-does-not-exist-abcxyz123/no-way