Audio: Surface proximity ticking + Voice settled-change; stage Edge staircase

Replaces the hands-free audio assist after extensive on-device iteration
(full journey + rejected approaches in AUDIO_DESIGN_LOG.md; current design
in SOUND_DESIGN_HANDOFF.md, from Jay's sound-design sessions).

Surface (TONES) = continuous proximity ticking: tick.wav repeats faster as
you near level (axis-agnostic warmer/colder, the rate is the message).
- Exponential rate in stable tilt error, clamped 1.5/s (>=~5 deg) to 8/s
  (<=~0.4 deg), hard-capped; constant loudness (acceleration is the signal).
- Deadline scheduler off a monotonic clock (SystemClock.elapsedRealtime):
  advances a next-tick deadline, so no drift and no catch-up bursts; <=1 tick
  per sensor update; immediate tick on re-entry (enable/return FACE_UP/unlock).
- Lock reuses LockDetector.isLocked (dwell + hysteresis, no second audio
  threshold): stop ticking, play level.wav once, silence while held. The
  1.39s level stream is retained and stopped on unlock/disable so a fresh
  tick can't overlap its tail.
- Pure SurfaceTickPolicy (unit-tested: anchors/clamps, monotonic rates,
  gating, immediate re-entry, no catch-up burst, level-once, resume on unlock).

Voice (VOICE) = settled-change announcer: one correction on the dominant
axis, silent while moving, speaks again on settle only if direction changed,
crossed coarse->fine, or reached lock. No periodic repeat. (VoiceGuidancePolicy,
unit-tested.)

Assets: tick.wav + level.wav downsampled to mono 44.1kHz in res/raw (from
Jay's SoundQ-derived 96k masters). AudioAssistMode adds OFF/TONES/VOICE.

Retired the settle-gated two-ding packet scheduler (AudioAssistStateMachine +
its ding assets) - superseded by ticking for Surface; logged as tested/rejected.

Edge staircase (1-D, future): 5 pitch-contour masters staged in
sonar-staircase-v3/ for when Edge mode is built.

76 tests passing; assembleDebug clean; blessed on-device.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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