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.

tests/helpers.ts

@@ -29,6 +29,7 @@ import type { Position, PropertyBag, Vec2 } from '../src/core/model/Properties';
 import { createEmptyProject, findScene, type Project } from '../src/core/model/Project';
 import type { SceneObject } from '../src/core/model/SceneObject';
 import { deserializeProject, serializeProject } from '../src/core/store/serialize';
+import type { AudioSink } from '../src/runtime/systems/Audio';
 import { BehaviorSystem } from '../src/runtime/systems/BehaviorSystem';
 import { CollisionSystem } from '../src/runtime/systems/CollisionSystem';
 import { AnimationSystem } from '../src/runtime/systems/AnimationSystem';
@@ -193,6 +194,9 @@ export type FakeContext = NodeContext & {
   logs: string[];
   stopped: boolean;
   changes: { machineId: string; from: string; to: string }[];
+  /** What the context was asked to play and to silence, in order – nothing here makes a noise. */
+  played: { assetId: string; volume: number; loop: boolean | undefined }[];
+  silenced: string[];
 };
 
 /** A NodeContext double: self/owner handles, `speed = 100`, dt 0.5, moves applied directly, `log` and `stop` recorded. */
@@ -221,6 +225,10 @@ export function fakeContext(overrides: Partial<NodeContext> = {}): FakeContext {
       return { blockedX: false, blockedY: false };
     },
     log: (message) => logs.push(message),
+    played: [],
+    silenced: [],
+    playSound: (assetId, volume, loop) => ctx.played.push({ assetId, volume, loop }),
+    stopSound: (assetId) => ctx.silenced.push(assetId),
     spawn: () => null,
     destroy: () => {},
     stop: () => {
@@ -259,6 +267,8 @@ export interface WorldOptions {
   collisions?: boolean;
   animations?: boolean;
   behaviors?: boolean;
+  /** Where `Play sound` ends up; without one the run plays to nobody (see `SILENT_AUDIO`). */
+  audio?: AudioSink;
 }
 
 export interface StartedWorld {
@@ -285,7 +295,7 @@ export function startWorld(fixture: SceneFixture, options: WorldOptions = {}): S
   if (collisions) world.addSystem(collisions);
   if (animations) world.addSystem(animations);
   const behaviors = options.behaviors
-    ? new BehaviorSystem(fixture.project, input, { collisions, animations })
+    ? new BehaviorSystem(fixture.project, input, { collisions, animations, audio: options.audio })
     : null;
   if (behaviors) world.addSystem(behaviors);
   return {