CodeRabbit and FredApps both flagged that the preflight probed only the plain
PKG_CONFIG variable, while the pkg-config crate alsa-sys pins resolves the
target-scoped variants first and falls back pkg-config -> pkgconf; on a host
with only pkgconf the ci stage failed a build that would succeed. The probe
now goes through the same crate (0.3.34, already pinned via alsa-sys), so the
semantics match by construction, and a probe that ran but said no stays
distinct from one that could not run.
Also from review: audio_backend_hint is gated to Linux (bflatastic: dead code
on Windows/macOS under -D warnings), the fake-tool regression runs the real
xtask audio-backend task in a child process with a controlled environment
(pkgconf-only host, a probe that says no, one that cannot run, a HOST_PKG_CONFIG
override), the rerun-if-env lines stay out of the ci log, and CONTRIBUTING.md
asks for a usable alsa.pc and runtime library rather than development headers.
Runs every scenario N times (default 1, unchanged). Every run is still
appended to results.jsonl; with N > 1 one "kind": "summary" row per key
follows, whose metrics are the medians of the successful runs (user_s and
sys_s included, byte/instruction counts rounded) and whose summary holds
median, p90 (nearest rank, = max for N < 10), min and max. A failed run marks
the summary failed so the gate refuses it.
--compare uses the latest row per key; the table labels each side "median
of N" or "1 run" and warns when they differ, and --fail-over refuses such
keys. --repeat with --with is rejected. docs/testing.md documents the format.
