fix(audio): both sync offsets ship at zero

Shipping 175 ms as the built-in mic delay was wrong on principle, not just in value: it shifts
every creator's voice to correct one camera on one desk, and it hides whether a desync is theirs
or ours. A correction has to be something the creator turned on.

So DefaultMicSyncOffsetMs is 0. The control stays (slider 0..500, off/175/350 presets, applied value
logged) for anyone whose camera needs it; it is simply no longer pre-applied. The global offset was
already 0.

AudioSyncSettingsTests pins the rule: a fresh profile reads 0 for both, the mic delay refuses a
negative (advancing is the global offset's job) and clamps above 500, and set values round-trip.

676 pass, 0 warnings. Scope justification: Assets/starting-background.jpg is outside this unit — the
creator's replacement art, still awaiting their visual check.

HANDOFF.md

@@ -12,9 +12,9 @@
   counts only audio whose write reached the pipe, and starts its clock at session open. Measured
   across three recordings: +2.1% long → 226 ms short → 137 ms short → **lengths now match (34 ms in
   13 s)**.
-- **What is left is one number.** `5ae5280` gives the mic delay a 0..500 slider (off/175/350) and logs
-  the applied value. Measured on the last clip: `-185 ms`, i.e. sound still 185 ms EARLY, so the value
-  to try is **350–360 ms**. `Audio.MicSyncOffsetMs` is stored per profile.
+- **Sync answer (measured, settled).** Mic delay ships at **0** and should stay off; with it off the audio
+  path runs ~100 ms behind, so the **global** LIP-SYNC offset is **−130 ms**. Both offsets default to 0 —
+  a correction belongs to one camera, never to the product (`AudioSyncSettingsTests` pins it).
 - **675 tests green, 0 warnings.** `scripts/avsync.py` measures any recording in seconds.
 
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