feat: the extension adds Discord channels to an artist you pick, and its tests gate the XPI (milestone 429)

Server (#4420)
- extension_service gains a Discord pattern (server or channel, jump links,
  ptb/canary; not DMs or threads), mirrored in platforms.js and pinned by
  the shared artist-url-samples.json.
- probe on a Discord URL matches the source by ids under any artist, reports
  a whole-server source as covering the channel, suggests the artist who owns
  another source on the same server, and names server/channel via the stored
  token (best-effort, bounded, no rate-limit waits).
- quick-add takes artist_id / artist_name; Discord URLs are stored canonical.

Extension (#4421, #4422)
- Content script on discord.com; SPA navigation by URL polling (the old
  pushState patch ran in the isolated world and never fired); stale probes
  are dropped.
- Discord chip opens an Add panel: this channel or the whole server, and the
  suggested artist / a search / a new name.
- Popup: sources show artist, platform and state; a Discord token export is
  verified by FC and the result shown. Token capture covers ptb/canary.
- Pure logic in lib/chip.js and lib/popup-format.js, with specs.

CI (#4423)
- extension.yml's lane (web-ext lint, vitest, XPI contents) moves into
  build.yml as extension-test and joins the needs of sign-extension,
  build-web and build-agent. As a separate workflow it gated nothing: a red
  extension suite still signed and shipped the XPI (rule 177).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

extension/lib/chip.js

@@ -0,0 +1,80 @@
+/**
+ * The content script's decisions, kept apart from its DOM so the specs can
+ * load them (test/chip.spec.js): which state the chip shows, what it says,
+ * and what the Discord Add panel starts out proposing.
+ *
+ * `probe` is /api/extension/probe's answer; `platformName` is the display
+ * name (PLATFORMS[key].name), passed in so this file needs no other lib.
+ */
+
+function chipState(probe) {
+  if (!probe || probe.error) return 'new';
+  return ({ source_match: 'source-match', artist_match: 'artist-match', new: 'new' })[probe.state] || 'new';
+}
+
+function chipLabel(probe, platformName) {
+  if (!probe || probe.error) return '+ Add to FabledCurator';
+  const artist = probe.artist?.name || 'artist';
+  if (probe.platform === 'discord') {
+    const d = probe.discord || {};
+    if (probe.state === 'source_match') {
+      return probe.covered_by_server
+        ? `✓ Whole server in FabledCurator · ${artist}`
+        : `✓ In FabledCurator · ${artist}`;
+    }
+    // Every other Discord state opens the panel: the artist is always chosen.
+    return d.channel_id ? `+ Add ${channelLabel(d)} to FabledCurator` : '+ Add server to FabledCurator';
+  }
+  switch (probe.state) {
+    case 'source_match':
+      return `✓ In FabledCurator · ${platformName}`;
+    case 'artist_match':
+      return `+ Add ${platformName} source to ${artist}`;
+    default:
+      return '+ Add to FabledCurator';
+  }
+}
+
+/** `#name` when the probe could read it, else a neutral "this channel". */
+function channelLabel(d) {
+  return d.channel_name ? `#${d.channel_name}` : 'this channel';
+}
+
+/** `name` when the probe could read it, else "this server". */
+function serverLabel(d) {
+  return d.server_name || 'this server';
+}
+
+/**
+ * What the Discord Add panel opens with. The channel is the default scope
+ * when there is one: a server source walks every channel the token can read,
+ * which is rarely what a single art channel wants. The artist is the probe's
+ * suggestion (the owner of another source on this server), else a new artist
+ * named after the server.
+ */
+function discordPanelDefaults(probe) {
+  const d = probe?.discord || {};
+  const suggested = probe?.state === 'artist_match' && probe.artist ? probe.artist : null;
+  return {
+    scope: d.channel_id ? 'channel' : 'server',
+    channelUrl: d.channel_url || null,
+    serverUrl: d.server_url || null,
+    artist: suggested ? { id: suggested.id, name: suggested.name } : null,
+    artistName: suggested ? suggested.name : (d.server_name || ''),
+  };
+}
+
+/**
+ * The quick-add body for the panel's current choice. A picked artist goes by
+ * id — names can collide once slugified — and a typed name creates (or
+ * finds) that artist. null when there is nothing valid to send.
+ */
+function discordAddRequest(choice) {
+  const url = choice.scope === 'server' ? choice.serverUrl : choice.channelUrl;
+  if (!url) return null;
+  if (choice.artist && choice.artist.id != null && choice.artist.name === choice.artistName) {
+    return { url, artistId: choice.artist.id };
+  }
+  const name = (choice.artistName || '').trim();
+  return name ? { url, artistName: name } : null;
+}