Slowed playback: no stutter, no doubled kicks, no echo

[thcp]

Closes #722
Closes #728
Closes #729

Reported and diagnosed by @goermezer in #701. This takes the engine half of their patch, with two adjustments. The slider half stays open in #701 as a separate design question.

What changes

The chunk scheduler's lookahead gate now measures against ctxTimeToSourceTime(ctx.currentTime) instead of the output playhead.

• SoundTouch path: sources play at 1x, so the gate now counts at 1x. Below 1x it no longer overstates its margin, and the sources stay fed.
• Tape-effect fallback: ctxTimeToSourceTime follows _srcRate(), so the gate still counts at the slowed rate there, the same as before.
• 1x: unchanged when nothing is transposed. With a transpose it schedules the pipeline latency (a fraction of a second) earlier, which is harmless.

Differences from the patch in #701:

1. It reuses the existing ctxTimeToSourceTime instead of adding a second clock that assumed 1x, which was wrong for the tape-effect fallback.
2. It does not subtract the pipeline latency, since that delay sits after the sources.

Tests

tests/js/slowed-scheduling.test.mjs plays a two minute synthetic track against a fake clock and checks, every 100 ms, that the scheduled sources still reach past "now". It covers SoundTouch at 1x, SoundTouch at 0.75x and tape effect at 0.75x.

• Without the fix, SoundTouch at 0.75x runs dry at 50.1 s.
• With it, all 9 checks pass.

The e2e fixture is 6 seconds long, so no Playwright test can reach this.

Doubled kicks at slowed speeds (#728)

Found while testing the fix above. Below 1x the stretcher's grains overlap, and an attack in the overlap is played twice, which turns a kick into a ~50 ms flam.

• The shared tempo stage marks attacks and, when the next grain would replay one, carries straight on instead. The splice joins identical samples, so it is seamless.
• The time that costs is borrowed, at most 100 ms, and paid back over the next few grains. The whole mix and the click share this stage, so they stay together. Only the playhead can trail the audio, by up to 100 ms, briefly.
• The pitch stages do not do this. They are aligned sample for sample with the unpitched drums.

Measured at 0.75x on 30 s of a full mix: 93 attacks in, 104 out before, 93 after.

tests/js/tempo-attacks.test.mjs stretches 40 synthetic kicks over a held chord. The old stretcher gives 47 attacks, the new one exactly 40, and no kick drifts past the borrow limit.

Signalsmith Stretch as the tempo stage (#729)

Below 1x, WSOLA repeats overlapping 82 ms fragments, which is heard as an echo on everything sustained, not only drums. The shared tempo stage now uses the Signalsmith Stretch WASM core (MIT), at 40 ms blocks. That size was chosen by ear against 30, 60 and the library's 120 ms default. The 120 ms default softened about a quarter of the drum attacks.

• Vendored as static/vendor/signalsmith-stretch.js: the loader from signalsmith-stretch@1.3.2, unchanged, with its MIT license. The package's own AudioWorkletNode is not used, because it ignores the rate on live input.
• Falls back to WSOLA if the core does not load. The swap happens only at a flush, never under audio already playing.
• Latency is reported by the processor, and both engines compute it from one shared module, static/js/tempoStage.js. Measured end to end, it matches within 1 ms at 44.1 and 48 kHz.
• CSP: script-src gains 'wasm-unsafe-eval', in both the server and Tauri CSPs. It allows WebAssembly compilation only; JS eval stays blocked. test_csp.py now pins script-src to exactly those two sources.
• CPU: about 2.5% of one core at 44.1 kHz, offline.

On 20 s of a full mix at 0.75x (55 attacks in): WSOLA 60 attacks out with 8 close pairs, Signalsmith 61 with 2.

Tests:

• tests/js/signalsmith-stage.test.mjs: the swap, the fallback, latency within 1 ms, and 40 kicks out as 40 with no drift.
• tests/e2e/tempo-stage.spec.mjs: in real Chromium, slowed playback reports the Signalsmith stage. With 'wasm-unsafe-eval' removed it fails and reports wsola, which also shows the fallback working in a browser.

Found along the way, not changed here: the WSOLA path's latency as the engines count it (122 ms) is well under what I measure end to end (about 300 ms at 0.75x). Only the fallback still uses that path.

Gate

• node --check on all of static/js, every tests/js test, i18n coverage clean, uv.lock untouched.
• Playwright: 411 passed on the final run. retention-setting and lyrics-align each failed once in earlier runs and passed on rerun with and without these changes.
• pitch-shift.test.mjs still passes, so the pitch stages are unaffected.
• pytest: the 8 known machine-local ffmpeg failures only. ruff and bandit clean.
• Rust: cargo fmt --check, clippy -D warnings and 116 tests pass (tauri.conf.json changed).