feat(agent): session-ready webhook for SSE-driven player handshake (0.9.13)

Closes Fase 3.3b. Daemon now tells the server the moment a session's
first HLS segment + init.mp4 land on disk; the web side flips
streaming_session.ready_at = NOW(), which its SSE endpoint pushes to
subscribed players so the loading UI flips from "Preparando…" to
"Stream listo" without polling HEAD on the segment URL.

Surface:
  - New Client.MarkSessionReady(ctx, sessionId) HTTP method →
    POST /api/internal/agent/session-ready.
  - New engine.HLSSession.ReadyCount() + FromCache() accessors so the
    watcher goroutine doesn't reach into private state.
  - New cmd.watchSessionReady(ctx, client, hsess, sessionId) goroutine
    polls ReadyCount every 200 ms with a 60 s deadline + short-circuits
    for cache-HIT sessions (ready the moment StartHLSSession returns).
  - Daemon callback spawns it right after streamSrv.HLS().Register so
    the watcher's lifecycle matches the session's.

Best-effort: a transient network failure on the webhook is logged + the
goroutine exits — the player's existing HEAD-probe retry path still
discovers ready state independently. The webhook is an acceleration,
not a hard dependency.

internal/engine/hls.go

@@ -519,6 +519,21 @@ func (s *HLSSession) ProbeInfo() map[string]any {
 	}
 }
 
+// ReadyCount returns how many segments are currently fully on disk.
+// Caller can `>= 1` it to check whether seg-0 has landed (and so the
+// player can be told to attach). For cache-HIT sessions this is always
+// `segmentCount` from the moment StartHLSSession returns.
+func (s *HLSSession) ReadyCount() int {
+	s.readyMu.Lock()
+	defer s.readyMu.Unlock()
+	return s.readyMax
+}
+
+// FromCache reports whether this session was served from the HLS cache
+// (no ffmpeg subprocess spawned). Used by ready-watcher logic to short-
+// circuit polling — a cache HIT is ready the moment we return.
+func (s *HLSSession) FromCache() bool { return s.fromCache }
+
 // MasterPlaylist returns the rendered master.m3u8 contents.
 func (s *HLSSession) MasterPlaylist() string { return s.manifestRoot }