feat(audio): Campaign A slice A5 — retail's region ambient soundscape

acdream had no ambient system: StartAmbient minted a handle and played
nothing. Retail's is a weighted-accumulation + timer-queue engine, not
looping voices. On every objcell change (24 m) CellManager::ChangePosition
rebuilds per-sound weights over the 3x3 landblock ring x 64 land cells
each, decoding each cell's terrain word through the region file's
terrain -> scene -> AmbientSTBDesc chain; playback is a min-heap of
absolute deadlines drained from the frame tick, where each pop fires a
one-shot and re-arms.

A continuous bed (base_chance == 0) is non-positional, crossfaded by its
share of the TOTAL weight, and re-fired every min_rate seconds — that
rate is the author's intended loop period, and re-firing is how retail
fakes a sustained bed with no looping voice, re-rolling the variant and
the crossfade each time. An intermittent one keeps its authored volume,
plays at a random accumulated compass bearing at min + (max-min)*t^2,
and is dice-gated. Indoors is silent by design: CEnvCell's contributor is
a folded ret and EnvCell carries no sound data.

The Opus review caught four bugs before this landed, one fatal:

- Cell offsets were built in ABSOLUTE world coordinates and differenced
  against the listener's STREAMED-frame position, so every one of 576
  offsets came out ~32 km, every contribution was culled, and the whole
  feature was silent with nothing logged. Offsets are now landblock-local
  the way Position::get_offset builds them, and the streamed-frame
  position is carried separately for playback, where it belongs.
- The cell's weight was added to the shared denominator once per
  DESCRIPTOR instead of once per CELL, dividing every bed's crossfade by
  the table's entry count — enough to push a typical authored volume
  under the 0.03 audibility floor.
- The drain used  where retail's UseTime is strictly
  below, so a descriptor authored with a zero rate re-armed at the same
  instant and spun the frame forever.
- Arming only enqueued; retail's UpdatePlayQueue PLAYS and then re-arms,
  so a newly audible ambient was silent for a full period after the
  crossing that made it audible.

Also: beds now go through retail's single 16-voice priority pool rather
than acdream's UI pool (retail has one pool; parking beds in the UI pool
let an A4 portal cue chop one mid-wave and discarded the authored
priority), and CalcDir's in-block test is XY-only, since CalcWeight
includes Z on purpose and CalcDir excludes it on purpose.

Two behaviours are knowingly incomplete and registered rather than
guessed at slice end: TS-66 (sky-lit interiors should keep the outdoor
set) and TS-67 (contribution weight is computed in-plane). Retires TS-29.

The frame-loop hook is a typed IAmbientFramePhase, not a callback — the
first attempt used an Action<float> and the architecture guard
ExtractedUpdateOwners_DoNotRetainAnonymousCallbacks correctly rejected it.

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

docs/plans/2026-08-08-audio-parity-campaign.md

@@ -306,5 +306,5 @@ global kill switch.
 | A2 | **COMPLETE** 2026-08-08 | `6d0156cb` | 118 Core audio tests (mixer + voice pool + cookbook); full Release suite 11,639 passed / 4 skipped / 0 failed. Opus review run and applied — 2 HIGH (pan-law saturation, stale `FUN_00550ad0` header), 5 MEDIUM (untested clamp order / pan truncation / voice pool, dead `PlayingGain`, duplicated heading helper), 5 LOW. Retires AP-28; files AP-173, AP-174, TS-64, TS-65. **Owed: user listening gate.** |
 | A3 | **COMPLETE** 2026-08-08 | `3fae0c7d` | 14 wire-conformance tests + 5 controller tests; full Release suite 11,658 passed / 4 skipped / 0 failed. **Owed: connected gate** (melee hit / pickup / lifestone audible against ACE). |
 | A4 | **COMPLETE** 2026-08-08 | `489ea6ad` | UI bank DID resolved from the dats (`0x2000004B`, content-verified: exactly the 32 `UI_*` slots) + 21-case environ table, 30 new Core tests; full Release suite 11,691 passed / 4 skipped. Retires TS-54; narrows AP-115 to notice-only. **Owed: connected gate** (`@environs` thunder + recall cues audible). **Suite note:** two load-dependent measurement flakes were observed on separate full-suite runs (`RuntimeCollisionReportingStateTests.WarmedSteadyContactRefreshDoesNotAllocate`, and one unnamed Core.Net test); both pass in isolation and neither touches audio. |
-| A5 | — | — | — |
+| A5 | **COMPLETE** 2026-08-08 | `e81bc474` | 46 ambient conformance tests; full Release suite 11,739 passed / 4 skipped. Opus review run and applied — it caught a FATAL frame bug (cell offsets built in absolute world coordinates while the listener is in the streamed frame: every contribution culled at ~32 km, feature silent with no error), a per-entry vs per-cell denominator error that would have pushed multi-entry beds under the audibility floor, an infinite loop on a zero play-rate, and newly-audible ambients not firing until a full period later. Also moved beds onto retail's single 16-voice priority pool and made the in-block direction test XY-only. Retires TS-29; files TS-66 (`seen_outside` interiors), TS-67 (in-plane weight). **Owed: user listening gate.** |
 | A6 | — | — | — |