recordToWavPath on FluidMicrophone.start and FluidSystemAudio.start tees
the 16 kHz mono pipeline into an AVAudioFile 16-bit-PCM WAV writer at the
fan-out, ordered before every consumer so recording completeness never
depends on downstream sessions. Pure sink: never starts/stops the capture,
survives the system-audio watchdog rebuild (the fan-out carries it), and is
finalized on every real stop path — including watchdog-initiated stops and
rebuild failures. Unwritable paths fail start loudly; mid-write errors are
swallowed (reference-recorder parity). Naming/rotation/retention stay with
the caller. Example Live Mic tab gains a Record WAV toggle; native tests
cover the sink (finalized header, frame-exact length, int16 round-trip,
idempotent close) and the fan-out tee itself.