ext: content/ media detection (build order step 7)

Two independent signals feed one panel, per docs/05 §3:
- capture/media.ts (background): webRequest-based, recognizes .m3u8/.mpd
  URLs and the HLS/DASH content types, deduped per (tab, url) so an
  HLS live-refresh doesn't re-fire. media-bridge.ts relays a hit to the
  tab's content script over runtime.sendMessage, and separately answers
  the content script's media.listVariants/media.addVariant calls by
  forwarding them to the background page's transport.
- content/media-observer.ts: watches the page's own <video> elements
  (present now, added later, or with src changed) for the same
  extension signal, independent of what the network sniffer saw.

Either firing opens content/video-panel.ts's "Download this video ▾"
panel (built with createElement, matching the popup's innerHTML-free
approach), which lists variants from media.listVariants and greys out
any variant.drm or a wholly drmProtected manifest with "Protected
content" rather than attempting it. The extension still never parses a
manifest itself — that stays in the daemon, one language, one place.

content/index.ts is the manifest-registered entry (content_scripts in
manifest.json, added in the previous commit); build.mjs builds it as an
IIFE rather than ESM, since a manifest content script has no "type":
"module" declaration and an emitted top-level export would be a syntax
error there. tsconfig.json adds DOM.Iterable for NodeList iteration.

docs/05-extension-spec.md gets a short addendum (§8) documenting the
popup/options bridge and this media-detection split, since neither was
in the original design write-up.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Ed8KEmAW48v4YHdxLtqsMB

extension/src/background/media-bridge.ts

@@ -0,0 +1,55 @@
+// Wires capture/media.ts's manifest detection to the content scripts, and answers the
+// two calls the in-page video panel needs (media.listVariants, media.addVariant). A
+// separate, simpler channel from bridge.ts's port because content scripts are per-tab
+// and use one-shot runtime.sendMessage rather than a long-lived port.
+
+import type { MediaAddVariantParams, MediaListVariantsParams } from '../shared/protocol/index.js';
+import type { VeloxTransport } from './transport/index.js';
+import type { DetectedManifest } from './capture/media.js';
+
+export type ContentMessage =
+  | { type: 'velox-list-variants'; params: MediaListVariantsParams }
+  | { type: 'velox-add-variant'; params: MediaAddVariantParams };
+
+export type ManifestDetectedMessage = { type: 'velox-manifest-detected'; url: string; headers: DetectedManifest['headers'] };
+
+export interface TabsLike {
+  sendMessage(tabId: number, message: unknown): Promise<unknown>;
+}
+
+export interface RuntimeOnMessageLike {
+  addListener(cb: (msg: ContentMessage, sender: unknown, sendResponse: (r: unknown) => void) => boolean | void): void;
+}
+
+/** Tells the tab's content script a manifest was seen, swallowing the "no content
+ *  script listening" error a tab without one (or the daemon's own tab) throws. */
+export function notifyTab(tabs: TabsLike, manifest: DetectedManifest): void {
+  const message: ManifestDetectedMessage = { type: 'velox-manifest-detected', url: manifest.url, headers: manifest.headers };
+  tabs.sendMessage(manifest.tabId, message).catch(() => undefined);
+}
+
+export class MediaBridge {
+  constructor(private readonly getTransport: () => VeloxTransport | undefined) {}
+
+  attach(onMessage: RuntimeOnMessageLike): void {
+    onMessage.addListener((msg, _sender, sendResponse) => {
+      if (msg.type !== 'velox-list-variants' && msg.type !== 'velox-add-variant') return undefined;
+      void this.handle(msg).then(sendResponse);
+      return true; // sendResponse is called asynchronously
+    });
+  }
+
+  private async handle(msg: ContentMessage): Promise<{ ok: true; result: unknown } | { ok: false; error: string }> {
+    const t = this.getTransport();
+    if (!t) return { ok: false, error: 'transport not ready' };
+    try {
+      const result =
+        msg.type === 'velox-list-variants'
+          ? await t.call('media.listVariants', msg.params)
+          : await t.call('media.addVariant', msg.params);
+      return { ok: true, result };
+    } catch (e) {
+      return { ok: false, error: e instanceof Error ? e.message : String(e) };
+    }
+  }
+}