refactor(core): route the graph's enabled-node filter through one helper

`enabledAudioFxNodes()` existed and was used only by the engine, while
audioFxGraph restated the same predicate three times in three shapes —
`.filter((n) => n.enabled !== false)` in `shapeOf`, `if (node.enabled === false)
continue` in `buildFxChain`, and the filter again in `update`. All three now
call the helper.

Also drops the `shape = shapeOf(next)` at the end of `update`. The early return
above it already established that `shapeOf(next)` equals `shape`, so this was a
full normalise-and-join of the chain per observer tick to write back the string
that was already there. `shape` is `const` now, which is the compiler enforcing
the same thing.

Consolidating found a genuine gap: `update`'s filter had NO test. A bypassed
node is not in the graph, so the update must walk the ENABLED nodes to stay
aligned with what was built — walking every node shifts each one's parameters
into its neighbour, and the shape is identical either way so nothing forces a
rebuild that would expose it. Both other call sites failed under mutation; this
one passed. Now covered, and falsified: the lowpass keeps its old cutoff while
the bypassed peaking's values land on it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

packages/core/src/audio/audioFxGraph.test.ts

@@ -216,6 +216,39 @@ describe("buildFxChain", () => {
     expect(c.created.find((x) => x.kind === "biquad")!.frequency.value).toBe(2000);
   });
 
+  it("lines new values up against the built nodes, skipping a bypassed one", () => {
+    // A bypassed node is not in the graph, so the update has to walk the
+    // ENABLED nodes to stay aligned with what was built. Walking `next.nodes`
+    // instead shifts everything after the bypass by one and pushes each node's
+    // parameters into its neighbour — and the shape is unchanged either way,
+    // so nothing forces a rebuild that would hide it.
+    const c = ctx();
+    const withBypass = (frequency: number): HfAudioFxChain => ({
+      version: 1,
+      nodes: [
+        {
+          type: "highpass",
+          enabled: true,
+          params: { ...defaultAudioFxParams("highpass"), frequency: 100 },
+        },
+        { type: "peaking", enabled: false, params: defaultAudioFxParams("peaking") },
+        {
+          type: "lowpass",
+          enabled: true,
+          params: { ...defaultAudioFxParams("lowpass"), frequency },
+        },
+      ],
+    });
+    const h = buildFxChain(asCtx(c), withBypass(8000));
+    const biquads = c.created.filter((n) => n.kind === "biquad");
+    expect(biquads).toHaveLength(2);
+
+    expect(h.update(withBypass(3000))).toBe(true);
+    // The lowpass took the new cutoff; the highpass was left where it was.
+    expect(biquads[0]!.frequency.value).toBe(100);
+    expect(biquads[1]!.frequency.value).toBe(3000);
+  });
+
   it("reports that a rebuild is needed when the chain shape changes", () => {
     const c = ctx();
     const h = buildFxChain(asCtx(c), chain("peaking"));

packages/core/src/audio/audioFxGraph.ts

@@ -9,6 +9,7 @@
  */
 
 import {
+  enabledAudioFxNodes,
   getAudioFxDef,
   normalizeAudioFxParams,
   type HfAudioFxChain,
@@ -480,8 +481,7 @@ export interface FxChainHandle {
  * into the running nodes; when it changes, the caller rebuilds.
  */
 function shapeOf(chain: HfAudioFxChain): string {
-  return chain.nodes
-    .filter((node) => node.enabled !== false)
+  return enabledAudioFxNodes(chain)
     .map((node) => {
       const p = normalizeAudioFxParams(node.type, node.params);
       const poles = p.poles !== undefined ? `:${p.poles}` : "";
@@ -506,8 +506,7 @@ export function buildFxChain(ctx: BaseAudioContext, chain: HfAudioFxChain): FxCh
   const handles: { id?: string; type: string; handle: FxNodeHandle }[] = [];
 
   let tail: AudioNode = input;
-  for (const node of chain.nodes) {
-    if (node.enabled === false) continue;
+  for (const node of enabledAudioFxNodes(chain)) {
     const handle = buildFxNode(ctx, node.type, node.params ?? {});
     tail.connect(handle.input);
     tail = handle.output;
@@ -515,7 +514,7 @@ export function buildFxChain(ctx: BaseAudioContext, chain: HfAudioFxChain): FxCh
   }
   tail.connect(output);
 
-  let shape = shapeOf(chain);
+  const shape = shapeOf(chain);
 
   return {
     input,
@@ -523,8 +522,7 @@ export function buildFxChain(ctx: BaseAudioContext, chain: HfAudioFxChain): FxCh
     nodes: handles,
     update(next) {
       if (shapeOf(next) !== shape) return false;
-      const active = next.nodes.filter((node) => node.enabled !== false);
-      active.forEach((node, i) => {
+      enabledAudioFxNodes(next).forEach((node, i) => {
         const held = handles[i];
         if (!held) return;
         held.handle.update(normalizeAudioFxParams(node.type, node.params));
@@ -537,7 +535,9 @@ export function buildFxChain(ctx: BaseAudioContext, chain: HfAudioFxChain): FxCh
         if (node.id === undefined) delete held.id;
         else held.id = node.id;
       });
-      shape = shapeOf(next);
+      // `shape` is not reassigned: the early return above already established
+      // that `shapeOf(next)` equals it, so recomputing was a whole normalise +
+      // join per observer tick to write back the string that was already there.
       return true;
     },
     dispose() {