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/test/artist-url-samples.json

@@ -134,5 +134,42 @@
       { "url": "https://www.hentai-foundry.com/pictures/popular", "why": "gallery listing, not a user" },
       { "url": "https://www.hentai-foundry.com/", "why": "site root" }
     ]
+  },
+
+  "discord": {
+    "match": [
+      {
+        "url": "https://discord.com/channels/111111111111111111/222222222222222222",
+        "slug": "111111111111111111/222222222222222222",
+        "why": "a channel: the slug is server/channel -- a Discord URL names a place, not a creator, so the artist is chosen in the Add panel"
+      },
+      {
+        "url": "https://discord.com/channels/111111111111111111",
+        "slug": "111111111111111111",
+        "why": "a whole server"
+      },
+      {
+        "url": "https://discord.com/channels/111111111111111111/222222222222222222/333333333333333333",
+        "slug": "111111111111111111/222222222222222222",
+        "why": "a message jump link still names its channel"
+      },
+      {
+        "url": "https://ptb.discord.com/channels/111111111111111111/222222222222222222",
+        "slug": "111111111111111111/222222222222222222",
+        "why": "the ptb and canary clients serve the same channels"
+      },
+      {
+        "url": "https://discord.com/channels/111111111111111111/222222222222222222/",
+        "slug": "111111111111111111/222222222222222222",
+        "why": "trailing slash is tolerated"
+      }
+    ],
+    "no_match": [
+      { "url": "https://discord.com/channels/@me", "why": "the DM list is not a source" },
+      { "url": "https://discord.com/channels/@me/222222222222222222", "why": "a DM is not a source" },
+      { "url": "https://discord.com/app", "why": "the app shell, no server open" },
+      { "url": "https://discord.com/channels/111111111111111111/222222222222222222/threads/444444444444444444", "why": "thread links are left to the manual Add form" },
+      { "url": "https://discord.com/servers/111111111111111111", "why": "a server-discovery page, not a channel" }
+    ]
   }
 }