fix(daemon): defensive IsClosed check in watchSessionReady poll loop

Closes the deferred bajo-priority item from the fase 3.3b critico.

Without this the watcher kept polling a torn-down HLSSession for up
to 60 s — fine in current code paths (Close always pairs with ctx
cancel which makes the select{} branch fire), but the function's
correctness then leaned on a caller invariant rather than its own
state check. Adding IsClosed() as a public wrapper around the
existing isClosed() lets the watcher detect any future
session-shutdown path (registry replace, idle sweep, internal kill)
without touching the unexported helper.

internal/engine/hls.go

@@ -534,6 +534,13 @@ func (s *HLSSession) ReadyCount() int {
 // circuit polling — a cache HIT is ready the moment we return.
 func (s *HLSSession) FromCache() bool { return s.fromCache }
 
+// IsClosed reports whether Close() has been invoked. Exposed (vs the
+// internal isClosed) so external watchers — the ready-webhook
+// goroutine in cmd/daemon.go — can short-circuit polling on a session
+// that was torn down through a different code path (registry replace,
+// idle sweep) without racing on the unexported helper.
+func (s *HLSSession) IsClosed() bool { return s.isClosed() }
+
 // MasterPlaylist returns the rendered master.m3u8 contents.
 func (s *HLSSession) MasterPlaylist() string { return s.manifestRoot }