fix(core): keep authored gain above unity off el.volume in the sandbox bridge (#3349)

Authoring a clip above unity gain throws at runtime today.

## What breaks

`MAX_AUDIO_GAIN_DB = 12` makes `data-volume` legal up to ~3.98. The sandbox runtime's volume bridge assigns the product straight to the element:

```ts
el.volume = clipVolume * volume;   // init.ts, onSetVolume
```

`HTMLMediaElement.volume` is spec-pinned to [0,1] and **throws `IndexSizeError`** outside it — verified in Chrome, and the test DOM agrees:

```
el.volume = 2  →  IndexSizeError: Failed to set the 'volume' property...
```

The throw lands inside a `for` loop over every media element, so it takes the rest of the loop with it: every clip after the boosted one keeps whatever volume it already had, while `state.bridgeVolume` says the change was applied. A composition with one boosted clip stops responding to the volume control for every clip authored after it.

## The fix

Clamp what the element receives. That is not lossy, because the element was never where the boost lived — the transport gets the authored gain unclamped, and this PR pins that half too:

- `syncRuntimeMedia` hands `onElementVolume` both the element's clamped volume **and** the authored gain, so the transport can have the boost the element cannot hold.
- `setElementVolume` keeps that gain on the per-element node, clamped only to `MAX_AUDIO_GAIN`.

Those two paths already worked; they were untested, and they are the reason clamping the element is the right half to clamp.

## Tests

- `init.test.ts` — a boosted clip followed by a quieter one, both seeded with sentinels, then the real `set-volume` control message. Asserts the boosted element lands at 1 **and** that the clip after it still gets its own volume, which is what a throw mid-loop strands.
- `media.test.ts` — the transport receives the authored gain while the element stays legal.
- `webAudioTransport.test.ts` — the per-element gain node keeps a boost above unity.

All three mutation-checked: removing the clamp reds the first, and clamping the gain at either transport seam reds the others.

## Provenance

This is the last unlanded piece of #3280. That PR was rebased onto current `main` and collapsed from +3050 to +944, of which everything except these lines is either already merged (#3308, #3309, #3333, #3339) or duplicated by the open #3306 and #3310. Cutting it out separately because the throw is live on `main` now and shouldn't wait behind a PR that is otherwise redundant.

packages/core/src/runtime/init.ts

@@ -3159,7 +3159,12 @@ export function initSandboxRuntimeModular(): void {
         if (!(el instanceof HTMLMediaElement)) continue;
         const parsed = parseFloat(el.dataset.volume ?? "");
         const clipVolume = Number.isFinite(parsed) ? parsed : 1;
-        el.volume = clipVolume * volume;
+        // `data-volume` carries authored gain, which goes above unity now that
+        // the ceiling is 12 dB — and `el.volume` is spec-pinned to [0,1], so
+        // assigning the product raw THROWS IndexSizeError and takes the rest of
+        // the loop with it. The element carries the legal part; the boost above
+        // unity belongs to Web Audio, which already has it from `setVolume`.
+        el.volume = Math.max(0, Math.min(1, clipVolume * volume));
       }
     },
     onSetMediaOutputMuted: (muted) => {