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/README.md

@@ -0,0 +1,38 @@
+# hxprobe
+
+Per-phase HTTP latency probe built on [httpx](https://www.python-httpx.org/),
+matching Go's `http.DefaultClient`: HTTP/2 negotiated via ALPN, redirects
+followed by default, connection pooling, and default TLS verification —
+while reporting a six-phase timing breakdown (DNS, TCP connect, TLS, TTFB,
+Transfer, Total).
+
+This is a fully self-contained project: no imports outside this directory,
+own `pyproject.toml`, own lockfile (`uv.lock`). It can be copied out of its
+parent repo and used standalone.
+
+See [docs/usage/hxprobe.md](../docs/usage/hxprobe.md) in the parent repo for
+full flag reference, example output, and design notes (redirect semantics,
+a TTFB-accuracy finding worth knowing about).
+
+## Quick start
+
+Requires [uv](https://docs.astral.sh/uv/).
+
+```sh
+uv sync                              # create .venv, install deps + lockfile
+uv run python -m hxprobe https://example.com
+uv run python -m hxprobe -v http://github.com   # verbose: shows HTTP/2, redirects, TLS, cert
+```
+
+## Development
+
+```sh
+uv run pytest tests -m "not integration" -v   # hermetic unit tests
+uv run pytest tests -m integration -v         # live tests (hits the real internet)
+uv run ruff check .                           # lint
+uv run ruff format .                          # format
+```
+
+From the parent repo's root, the equivalent Makefile targets are
+`make hx-run`, `make hx-test`, `make hx-test-integration`, `make hx-lint`,
+`make hx-fmt`, and `make hx-check` (lint + hermetic tests).