Change the sound FX parameters of a sound while it plays #315

Until now a value handed to a sound FX counted from the start of the
voice only. An engine whose pitch follows the speed, or a loop that
gets quieter, could not be built. Every play of Play sound now gives
out the number of its voice, and a new node lets the knobs of that one
voice glide to new values while it plays.

- Play sound has a new output voice: the number of this one play, or 0
  where nothing was played. Kept in a Number variable it reaches the
  voice from another event. Two plays of one sound are two voices.
- New action node Set sound FX parameters (exec, voice, fx params).
  It reaches exactly the voice it is given, a detached one too. A
  voice that has ended, or voice 0, is not reached.
- AudioSink.play returns the voice number and takes the plan and the
  values (FxVoice) instead of a finished chain. AudioSink.tune takes a
  voice number and new values. A name the voice's FX does not have is
  left out, one not given keeps its value.
- SoundLibrary hands out the number at once, before the bytes are
  there, and never twice. A voice still waiting for its bytes starts
  with the values handed in meanwhile.
- Every knob of a voice moves with setTargetAtTime (30 ms time
  constant). A knob whose value did not change is not sent again, so a
  tune every frame costs next to nothing.
- A pitch on a playing voice glides by playbackRate. A new WSOLA copy
  in the middle of a sound is not worked out, so the voice gets shorter
  or longer while it is moved. On a loop that is not heard.
- fxChainOf now gives a chain whose shape depends on the plan alone.
  A stage fed by a parameter stays at 0 dB, and a Mix fed by a
  parameter keeps both ways. A typed-in 0 or Mix at 0/1 is still left
  out by the compiler.
- Every way of a voice has a source of its own, also at the same
  pitch, so each can glide on its own. The pitched copy is shared.
- The tab keeps the numbers of what it plays, so moving a parameter
  field is heard at once.
- Test hook __gm.sound.levels(file) reads the knobs of every voice of
  a file, oldest first, and stats() counts the voices tuned.

Tests:
- soundfx.test.ts: 29 tests (+2 for the node in a running world: the
  voice of the first of two plays of one sound, and a play that played
  nothing)
- sound-library.test.ts: 15 tests (+3: glide, reach by number,
  pending voice)
- node-catalog.test.ts: 370 tests (+2, Play sound checks its voice)
- sounds + keep-on-scene-load: 31 tests, unchanged
- Counter-test: a tune that reaches every voice of the same file makes
  the reach test fail.
- e2e 34 (melektaus-atx): ALL PASSED - 30 checks (+1). In Chrome two
  looping voices of one sound start at depth 7. The one whose number
  sits in a variable glides to 0.7938 (2^(-4/12) = 0.7937), the other
  stays at 1. Both gains stay at 0.5012.
- The gate ran before voices got a number (CHECK EXIT 0, 241 files,
  3658 tests), not after.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core/node-catalog.test.ts

@@ -1940,12 +1940,14 @@ const CASES: Record<string, Case[]> = {
       name: 'no sound is a no-op that still continues the flow',
       inputs: { sound: '', volume: 1, loop: '' },
       next: 'exec',
+      outputs: { voice: 0 },
       after: (ctx) => expect(ctx.played).toEqual([]),
     },
     {
-      name: 'hands the id on with the volume and loop it was given',
+      name: 'hands the id on with the volume and loop it was given, and gives out the voice (#315)',
       inputs: { sound: 'snd-1', volume: 0.5, loop: 'true' },
       next: 'exec',
+      outputs: { voice: 1 },
       // What the two mean – a factor on the asset's volume, an answer that overrules its loop
       // flag – is the BehaviorSystem's to resolve, since only it holds the project (see
       // `sounds.test.ts`).
@@ -2025,6 +2027,20 @@ const CASES: Record<string, Case[]> = {
       after: (ctx) => expect(ctx.silenced).toEqual(['']),
     },
   ],
+  SetSoundFxParameters: [
+    {
+      name: 'hands the numbers of the bundle to the voice it is given',
+      inputs: { voice: 4, fxParams: { depth: 3, label: 'x' } },
+      next: 'exec',
+      after: (ctx) => expect(ctx.tuned).toEqual([{ voice: 4, fxParams: { depth: 3 } }]),
+    },
+    {
+      name: 'voice 0 – nothing was played – reaches nothing, and the flow carries on',
+      inputs: { voice: 0, fxParams: { depth: 3 } },
+      next: 'exec',
+      after: (ctx) => expect(ctx.tuned).toEqual([]),
+    },
+  ],
   Log: [
     {
       name: 'logs its message and continues',