status: consolidate CDN/upstream status into the header pill + tap panel

- pill label now shows KB/s when degraded ('cdn 23 KB/s'); tap opens a
  panel (replaces the transient toast) with origin/CDN state, KB/s,
  a sparkline of the ~1h /api/cdn hist, and last-updated age
- 60s poll while the page is visible (was: load/play/failure only)
- playback failure overlay appends CDN context when degraded/down
- buffer gauge + spinner untouched: client buffer state, not CDN state
- server unchanged: /api/cdn already returned everything
- tests: 3 hermetic browser tests (mocked /api/cdn via page.route —
  the throwaway server's probe runs live upstream, so its data is
  mocked out of the assertions); 37 passed
- docs: README + SPEC synced

SPEC.md

@@ -82,6 +82,9 @@ an exception falls through to the next provider, a `None` from
   position-event stream.
 - `GET /api/cdn` → CDN + origin health (background 64KB probe every
   3 min, ~1h history) — check whether "playback broken" is the CDN.
+  The header status pill is the single UI surface for this (tap opens
+  a panel: origin/CDN state, KB/s, sparkline of `hist`); the playback
+  failure overlay appends CDN context when degraded/down.
 - `GET /stream/{token}` → proxied HLS. Token = base64url of the
   upstream URL (host-allowlisted against the providers' self-registered
   CDN hosts — anti-SSRF). Rewrites every non-comment line of m3u8