fix(extension): trust HOST_BLOCKED as authoritative (join race, EC-5)

hcmHandleBlocked no longer re-checks local control mode before acting. Background
only sends HOST_BLOCKED to a gated guest, so the message itself is authoritative —
adopt host-only/guest role from it. Fixes the join race where a HOST_BLOCKED
arriving before the CONTROL_MODE broadcast was silently ignored.

Also corrects the EC-4 note: "let catch-up re-sync" is invalid (sync is
event-driven, no continuous catch-up); the right fix is a buffer-aware deferred
snap-back.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

extension/content.js

@@ -175,7 +175,12 @@
 
     // Entry point: background told us our local action was blocked in host-only.
     function hcmHandleBlocked(action, target) {
-        if (!hcmIsGuestGated()) return;
+        // HOST_BLOCKED is only ever sent to a gated guest (background verifies
+        // host-only + !host before sending), so it's authoritative. Adopt the
+        // role/mode from it in case our CONTROL_MODE broadcast hasn't landed yet
+        // (join race, EC-5) — otherwise we'd miss the dialog/snap-back.
+        hcmControlMode = 'host-only';
+        hcmAmHost = false;
         if (Date.now() < hcmSnapBackCooldownUntil) return; // EC-4 loop guard
         if (hcmDesynced) return; // already solo, nothing to do