fix(core): stop reading a blank FX parameter as zero

`normalizeAudioFxParams` coerced with a bare `Number(raw)`, and `Number(null)`,
`Number("")`, `Number(false)` and `Number([])` are all 0 — every one of them
finite. So a parameter that arrived missing or blanked clamped to 0 instead of
falling back to the effect's declared default, and because 0 is a legal setting
for most of these knobs nothing downstream could tell the difference: a
compressor whose threshold came through as null sat at 0 dB and never engaged.

Now only a number, or a string that actually spells one, counts — the same rule
`numberOrNull` in audioAutomation.ts already applies. Panel inputs arrive as
strings, so those still work.

Falsified by restoring the bare `Number(raw)`: null reads as 0 dB instead of
-24.

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

packages/core/src/audioFx.test.ts

@@ -69,6 +69,27 @@ describe("normalizeAudioFxParams", () => {
     expect(v.gain).toBe(0);
   });
 
+  it("treats a blank or missing value as absent rather than as zero", () => {
+    // `Number(null)`, `Number("")`, `Number(false)` and `Number([])` are all 0
+    // and all finite, so these used to clamp to 0 instead of falling back. Zero
+    // is a legal setting for most of these knobs, so nothing downstream could
+    // tell: a compressor whose threshold arrived as null sat at 0 dB and never
+    // engaged, silently, rather than at its declared -24 dB.
+    const def = defaultAudioFxParams("compressor").threshold;
+    expect(def).not.toBe(0);
+    for (const blank of [null, undefined, "", "   ", false, [], {}]) {
+      expect(
+        normalizeAudioFxParams("compressor", { threshold: blank as unknown as number }).threshold,
+        `${JSON.stringify(blank)} was read as a number`,
+      ).toBe(def);
+    }
+    // A string that really does spell a number still counts — that is how the
+    // panel's inputs arrive.
+    expect(
+      normalizeAudioFxParams("compressor", { threshold: "-30" as unknown as number }).threshold,
+    ).toBe(-30);
+  });
+
   it("falls back to the default for an unrecognised enum value", () => {
     expect(normalizeAudioFxParams("saturate", { type: "sawtooth" }).type).toBe("tanh");
     expect(normalizeAudioFxParams("saturate", { type: "atan" }).type).toBe("atan");

packages/core/src/audioFx.ts

@@ -761,7 +761,20 @@ export function normalizeAudioFxParams(
       out[p.key] = ok ? (raw as string) : p.default;
       continue;
     }
-    const n = typeof raw === "number" ? raw : Number(raw);
+    // Only a number, or a string that actually spells one. `Number(null)`,
+    // `Number("")`, `Number(false)` and `Number([])` are all 0 and all pass
+    // Number.isFinite, so a missing or blanked value used to clamp to 0 rather
+    // than fall back to the declared default — and 0 is a legal value for most
+    // of these knobs, so nothing downstream could tell. A compressor whose
+    // threshold arrived as null sat at 0 dB and never engaged, silently,
+    // instead of at its -24 dB default. `numberOrNull` in audioAutomation.ts
+    // already guards exactly this.
+    const n =
+      typeof raw === "number"
+        ? raw
+        : typeof raw === "string" && raw.trim() !== ""
+          ? Number(raw)
+          : Number.NaN;
     out[p.key] = Number.isFinite(n) ? Math.min(p.max, Math.max(p.min, n)) : p.default;
   }
   return out;