docs: close the audio-doctor decisions, spec is READYHEADmain

All eight decisions resolved. DOCTOR is a section-header key; CLI-first with
the GUI as a face; hung and dead are two verdicts sharing one remedy; the
remedy tiers stand as drafted; a stream-active guard refuses the audible
remedies and is overridable by pressing again; xruns stay out of v1.

Two consequences the questions did not make obvious. Taking the guard is what
lets the pipewire-pulse restart stay at Confirm — the danger lives in the state
of the machine, not the identity of the remedy. And the guard must read stream
state from pw-dump rather than pactl: the remedies it protects are the ones you
reach for when the Pulse layer is dead, so a pactl-fed guard would go blind
exactly when it is needed.

Tier 4 drops its xrun probe to match the decision, rather than leaving the
probe list contradicting the answer.

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