chore: checkpoint web Blender migration source, tools and docs

Accumulated uncommitted work: GHOST Web backend, WebGL/GPU web layer,
web workflow engine and protocol, tooling, docs and task records.
Large generated evidence, user-testing archives, build snapshots and
imported test assets remain untracked.

Co-Authored-By: Claude Code <noreply@anthropic.com>

web/app/src/sequencer/SequencerCodecProbe.ts

@@ -20,19 +20,23 @@ function blocked(
   return { ...request, status: "BLOCKED", backend, reason, decoded: null };
 }
 
-function waitForMedia(video: HTMLVideoElement, eventName: "loadeddata" | "seeked", timeoutMs: number): Promise<void> {
+function waitForMedia(video: HTMLVideoElement, eventName: "loadeddata" | "seeked", timeoutMs: number, signal?: AbortSignal): Promise<void> {
   return new Promise((resolve, reject) => {
     const timeout = window.setTimeout(() => finish(new Error("timeout")), timeoutMs);
     const finish = (error?: Error): void => {
       window.clearTimeout(timeout);
       video.removeEventListener(eventName, ready);
       video.removeEventListener("error", failed);
+      signal?.removeEventListener("abort", aborted);
       if (error) reject(error); else resolve();
     };
     const ready = (): void => finish();
     const failed = (): void => finish(new Error("decode"));
+    const aborted = (): void => finish(new DOMException("Movie probe cancelled", "AbortError"));
     video.addEventListener(eventName, ready, { once: true });
     video.addEventListener("error", failed, { once: true });
+    signal?.addEventListener("abort", aborted, { once: true });
+    if (signal?.aborted) aborted();
   });
 }
 
@@ -66,7 +70,8 @@ async function probeSound(request: SequencerCodecProbeRequestIR, data: ArrayBuff
   catch { return blocked(request, "WEB_AUDIO", "DECODE_FAILED"); }
 }
 
-async function probeMovie(request: SequencerCodecProbeRequestIR, data: ArrayBuffer): Promise<SequencerCodecProbeResultIR> {
+async function probeMovie(request: SequencerCodecProbeRequestIR, data: ArrayBuffer, signal?: AbortSignal): Promise<SequencerCodecProbeResultIR> {
+  signal?.throwIfAborted();
   if (typeof document === "undefined" || typeof HTMLMediaElement === "undefined") return blocked(request, null, "RUNTIME_UNAVAILABLE");
   const video = document.createElement("video");
   if (video.canPlayType(request.mimeType) === "") return blocked(request, "HTML_MEDIA", "MIME_UNSUPPORTED");
@@ -79,14 +84,14 @@ async function probeMovie(request: SequencerCodecProbeRequestIR, data: ArrayBuff
   try {
     video.src = url;
     video.load();
-    await waitForMedia(video, "loadeddata", 10_000);
+    await waitForMedia(video, "loadeddata", 10_000, signal);
     if (!Number.isFinite(video.duration) || video.duration <= 0 || video.videoWidth < 1 || video.videoHeight < 1) {
       return blocked(request, "HTML_MEDIA", "DECODE_FAILED");
     }
     const target = Math.min(video.duration / 2, Math.max(0, video.duration - 0.001));
     if (target > 0) {
       video.currentTime = target;
-      await waitForMedia(video, "seeked", 10_000);
+      await waitForMedia(video, "seeked", 10_000, signal);
     }
     const canvas = document.createElement("canvas");
     canvas.width = 1;
@@ -107,7 +112,10 @@ async function probeMovie(request: SequencerCodecProbeRequestIR, data: ArrayBuff
       },
     };
   }
-  catch { return blocked(request, "HTML_MEDIA", "DECODE_FAILED"); }
+  catch (error) {
+    if (signal?.aborted || error instanceof DOMException && error.name === "AbortError") throw error;
+    return blocked(request, "HTML_MEDIA", "DECODE_FAILED");
+  }
   finally {
     video.removeAttribute("src");
     video.load();
@@ -119,14 +127,19 @@ async function probeMovie(request: SequencerCodecProbeRequestIR, data: ArrayBuff
 export async function probeSequencerCodec(
   requestValue: unknown,
   source: ArrayBuffer,
+  signal?: AbortSignal,
 ): Promise<SequencerCodecProbeResultIR> {
+  signal?.throwIfAborted();
   const request = parseSequencerCodecProbeRequest(requestValue);
   if (!(source instanceof ArrayBuffer) || source.byteLength !== request.byteLength || await sha256(source) !== request.sourceSha256) {
     return blocked(request, null, "SOURCE_IDENTITY_MISMATCH");
   }
-  if (request.stripType === "IMAGE") return probeImage(request, source);
-  if (request.stripType === "SOUND") return probeSound(request, source);
-  return probeMovie(request, source);
+  signal?.throwIfAborted();
+  const result = request.stripType === "IMAGE" ? await probeImage(request, source)
+    : request.stripType === "SOUND" ? await probeSound(request, source)
+    : await probeMovie(request, source, signal);
+  signal?.throwIfAborted();
+  return result;
 }
 
 /** Probe a movie with the browser decoder first, then use the local ffmpeg.wasm
@@ -136,7 +149,9 @@ export async function probeSequencerMovieCodecWithFallback(
   source: ArrayBuffer,
   signal?: AbortSignal,
 ): Promise<SequencerCodecProbeResultIR> {
-  const browserResult = await probeSequencerCodec(requestValue, source);
+  signal?.throwIfAborted();
+  const browserResult = await probeSequencerCodec(requestValue, source, signal);
+  signal?.throwIfAborted();
   if (browserResult.status === "READY") return browserResult;
   const { probeMovieWithFFmpeg } = await import("./FFmpegMovieProbe");
   return probeMovieWithFFmpeg(requestValue, source, signal);