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/shared.ts

@@ -36,6 +36,17 @@ export const keyParam: PropertyDef = { key: 'key', label: 'Key', kind: 'key', de
 /** No `assetTypes`: any placeable asset is a valid answer here. */
 export const assetParam: PropertyDef = { key: 'asset', label: 'Asset', kind: 'asset', default: '' };
 
+/**
+ * The `sound` input of the two sound nodes; unconnected, it shows the sound picker – the way
+ * `machinePort` shows the machine picker. A `Sound` literal is what feeds it through an edge.
+ */
+export const soundPort = (): PortDef => ({
+  key: 'sound',
+  label: 'sound',
+  type: 'string',
+  field: { key: 'sound', label: 'Sound', kind: 'sound', default: '' },
+});
+
 export function binaryMath(type: string, description: string, fn: (a: number, b: number) => number): NodeDef {
   return {
     type,