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its ding assets) - superseded by ticking for Surface; logged as tested/rejected.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>

AUDIO_DESIGN_LOG.md

@@ -0,0 +1,76 @@
+# Audio Assist — design log
+
+Hands-free audio to level something without watching the screen. Setting cycles
+**OFF / TONES / VOICE** from the Level header. Plays on `USAGE_MEDIA` (so the volume slider
+controls it) and never takes audio focus.
+
+> **Source of truth for the current design is [`SOUND_DESIGN_HANDOFF.md`]**, from Jay's
+> dedicated sound-design sessions. This log records the *journey* — especially rejected
+> approaches — so they don't get rediscovered. Where the two ever disagree, the handoff wins.
+
+## Chosen direction (current)
+
+Two modes / two mechanisms, one sound identity (all derived from a 500 Hz blip):
+
+**SURFACE (2-D) — TONES = continuous proximity ticking.** A tick (`tick.wav`, 60 ms) repeats
+faster as you near level — axis-agnostic "warmer/colder", the RATE is the whole message (a
+bubble level asks nothing of you). **No settle gate** — it ticks at every point in the
+adjustment. Rate is exponential in the stable tilt error, clamped **1.5/s (≥~5°) → 8/s
+(≤~0.4°)**, hard-capped at 8/s. **Constant loudness** (acceleration is the signal, not volume).
+On lock (reusing `LockDetector.isLocked` — dwell + hysteresis, no second audio threshold):
+ticking stops, `level.wav` plays once, silence while it holds; unlock resumes ticking.
+Scheduling is deadline-based off a **monotonic clock** — advances a next-tick deadline (no
+drift, no catch-up bursts, ≤1 tick per sensor update); re-entry (enable TONES / return
+FACE_UP / unlock) ticks immediately. `level.wav`'s 1.39 s stream is retained and stopped on
+unlock/disable so a fresh tick can't overlap its tail.
+
+**EDGE (1-D, future) — the "sonar staircase."** Direction + magnitude as a 4-blip pitch
+contour (rising = raise it, falling = lower it, steepness = how far; level sounds flat), one
+report per move→settle. Five pre-rendered assets staged in `sonar-staircase-v3/`; wired when
+Edge mode is built.
+
+**VOICE — deliberately separate, a SETTLED-CHANGE announcer (not a timer).** Speaks one
+correction, then stays silent while moving; when it settles again it speaks only if the
+dominant direction changed, it crossed coarse→fine, or it reached lock ("that's level"). A
+settle still needing the same coarse nudge gets silence. No periodic repeat, no bed under it.
+
+Implementation: pure `SurfaceTickPolicy` (monotonic deadline scheduler, exponential rate,
+unit-tested) + `VoiceGuidancePolicy`; the SoundPool layer stays dumb (play a tick / play &
+stop level). Sounds are Jay's SoundQ-derived blips (`tick.wav`, `level.wav`) downsampled to
+mono 44.1 kHz in `res/raw`.
+
+## Rejected approaches (and why)
+
+1. **Two fixed proximity bands + lock ping** (original). Worked but coarse; the near/close jump
+   was audible ("a lower ding when close").
+2. **Continuous cadence** — beep interval shrinks smoothly toward center (parking-sensor).
+   → "gets a bit much." Monotonous over a long adjustment.
+3. **Rising-pitch discrete beeps.** Better, but still same-note-ish and busy.
+4. **Continuous glide tone** — one warm tone, pitch low→treble by closeness, resolving into the
+   aura. → "the constant sound works but it's too much; not clicking." A continuous tone gives
+   the ear **no silence to rest on**, so it fatigues; it also bypassed the tested scheduler.
+5. **Radar doublet on a per-band cadence** — two sonar pings ~110 ms apart, repeating on a
+   1.3/0.9/0.6 s cadence while off-level. The literal two-ping "pew-pew" read as UI/radar beeps,
+   not sonar, and the continued cadence was still a clock. Kept the bands/pitch, dropped the
+   cadence in favor of one packet per settle.
+6. **Continuous forcefield aura bed** (looping CC0 ambience while locked). Seamless on a PC
+   media player but **clicked at the loop point through Android SoundPool**. Pulled; a gapless
+   bed needs AudioTrack/ExoPlayer.
+7. **Settle-gated two-ding "status packet"** (banded pitch + gap, one packet per move→settle,
+   fuller arrival ding at lock). Built, unit-tested, on-device tested. Coherent, but on the
+   bench the settle-then-report model felt sparse/laggy for the fiddly 2-D surface adjustment —
+   you want live "warmer/colder" while you're actually moving it, not a report after you stop.
+   Superseded by continuous ticking for Surface. (The settle-then-report shape lives on for
+   VOICE and for future EDGE, where a discrete per-settle report fits the 1-D task.)
+
+## Guiding principles (learned)
+
+- Leave **air** between cues; silence is restful. (600 ms near cadence is responsive without
+  being alarm territory; ~180 ms was alarm territory.)
+- **Pitch** carries distance more reliably than **volume** (phone volume + room noise make
+  loudness a poor information channel).
+- Keep the SoundPool layer dumb; put decisions in tested pure logic.
+- The **centered** sound needs source loudness/spectrum presence, not just a bigger volume
+  number.
+- No reverb/comb tails on phone speakers — they smear and lengthen fatigue.
+- Reuse the real lock state for the centered handoff; never invent a second audio threshold.