fix: AbortSignal now sends CANCEL to stop running exec (#719)

[JacobiusMakes]

Summary

Fixes #719. When AbortSignal fires during exec() or ffprobe(), a CANCEL message is now sent to the worker to actually stop the running FFmpeg command — not just reject the JS Promise.

Problem

Previously, aborting an exec() or ffprobe() only rejected the JavaScript Promise. The underlying WASM process continued running to completion, wasting CPU and making AbortController useless for long transcoding operations.

Fix

const.ts — Add FFMessageType.CANCEL to the message enum.

classes.ts — When the abort signal fires for EXEC or FFPROBE, send a CANCEL message to the worker (in addition to rejecting the Promise). Also fixes a memory leak: Blob URLs created for coreData/wasmData/workerData are now tracked in #blobURLs and revoked in terminate().

worker.ts — Handle CANCEL by calling ffmpeg.setTimeout(1), which sets the WASM-side watchdog to fire after 1ms. This is the same mechanism used by the existing timeout parameter — it stops the current command and returns exit code 1.

Known limitation

In single-thread mode, exec() blocks the worker's JS event loop while running WASM. The CANCEL message queues behind it and only executes after the command finishes. For true mid-execution cancellation, the multi-thread build (which uses SharedArrayBuffer) would be needed. This PR still improves the situation by:

1. Rejecting the Promise immediately (existing behavior, preserved)
2. Sending CANCEL so the next queued operation doesn't run
3. Fixing the blob URL memory leak

Test plan

• Abort an exec() before it starts — CANCEL arrives first, sets timeout to 1ms, exec returns immediately
• Abort during a long transcode — Promise rejects, CANCEL queues
• Call terminate() after using coreData/wasmData — verify no blob URL leak via DevTools memory panel
• Existing test suite passes

🤖 Generated with Claude Code

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