A game that stays silent is only half a game, so a sound is an asset like any other (#45)

Sounds follow the road fonts already paved: the file goes to the server
content-addressed under its sha256, the project keeps a named asset that
points at it, and nothing is ever re-encoded. The format is read from the
file's own signature, not from what the browser claims - the same .m4a
arrives as audio/mp4, audio/x-m4a or as nothing at all.

Playing is two nodes. "Play sound" takes the asset and may raise or lower
its volume; every call is its own voice, so two jumps in a row are two
jumps heard. Its loop input is a choice of three - from the asset, on,
off - because a checkbox has to spend one of its two positions on "not
set" and would lose a case: an asset that loops could then never be
played once.

Below the graph core stays browser-free: NodeContext hands out an asset
id, BehaviorSystem resolves it against the project and folds the asset's
own volume and loop into the call, and only viewport/SoundLibrary knows
about Web Audio. A run without a sink plays nothing instead of throwing.

The CI browser is muted, so the ear is no witness: the debug hook counts
started and stopped voices, and step 34 reads that counter.

src/core/graph/nodes/index.ts

@@ -3,10 +3,12 @@ import { Destroy } from './action/Destroy';
 import { Emit } from './action/Emit';
 import { Log } from './action/Log';
 import { Move } from './action/Move';
+import { PlaySound } from './action/PlaySound';
 import { RestartGame } from './action/RestartGame';
 import { SetPosition } from './action/SetPosition';
 import { Spawn } from './action/Spawn';
 import { StopGame } from './action/StopGame';
+import { StopSound } from './action/StopSound';
 import { Abs } from './data/Abs';
 import { Add } from './data/Add';
 import { And } from './data/And';
@@ -32,6 +34,7 @@ import {
   BooleanLiteral,
   ListLiteral,
   NumberLiteral,
+  SoundLiteral,
   StateMachineLiteral,
   StringLiteral,
   TextureLiteral,
@@ -146,6 +149,7 @@ export const BUILT_IN_NODES: readonly NodeDef[] = [
   StringLiteral,
   TextureLiteral,
   StateMachineLiteral,
+  SoundLiteral,
   Add,
   Subtract,
   Multiply,
@@ -195,6 +199,8 @@ export const BUILT_IN_NODES: readonly NodeDef[] = [
   Spawn,
   Destroy,
   Emit,
+  PlaySound,
+  StopSound,
   Log,
   StopGame,
   RestartGame,