Player: remote control while casting

Shows the receiver's position with play/pause, ±30 s and a scrubber;
a seek restarts the relay where asked.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

2 files changed, 16 insertions, 1 deletions

diff --git a/docs/MESHBAY_DESIGN.md b/docs/MESHBAY_DESIGN.md
index 58bbbdb..e638f9b 100644
--- a/docs/MESHBAY_DESIGN.md
+++ b/docs/MESHBAY_DESIGN.md
@@ -3410,6 +3410,18 @@ old film's — the local player never noticed, a relay restarted there did. The
 local element keeps playing while a cast runs, because its playhead paces the
 stream, and is muted.
 
+**While a receiver plays the film, the player is its remote.** Phone and
+television start apart and drift, so a scrubber on the local playhead lied about
+where the film was and a seek from it landed off by the gap. The page polls
+`cast:status`, whose `chromecast` carries the receiver's `playerState` and
+`position` (stream seconds, zero at the relay's start; the page adds the
+stream's start), and shows that over the local picture with play/pause
+(`cast:chromecast:pause`/`play`), ±30 s and a scrubber. A seek is the ordinary
+seek: it restarts the relay and reloads the receiver there, and until that
+lands the target is shown, not the old stream's position. Pausing the receiver
+pauses the local element too, which otherwise goes on fetching for nobody. The
+copy-URL cast has no receiver to read and keeps the ordinary player.
+
 **Subtitles are rebased onto the relay's clock before they are sent.** The node
 extracts a track whole, so its cues carry the film's timeline, and the player
 can use them unchanged because its SourceBuffer is given `timestampOffset =
diff --git a/docs/USERGUIDE.md b/docs/USERGUIDE.md
index ad51982..1790475 100644
--- a/docs/USERGUIDE.md
+++ b/docs/USERGUIDE.md
@@ -395,7 +395,10 @@ resume.
 **The desktop and Android applications.** A film playing in the application can
 be sent to a cast-capable TV or dongle on the same network: *Cast to device* in
 the player, pick one from the list. On a phone, the film goes on playing silently
-on the phone while the TV shows it, and the screen can be turned off. Devices appear as they answer, usually within a couple
+on the phone while the TV shows it, and the screen can be turned off. While a
+TV plays the film, the player becomes its remote: the position shown is the
+TV's, with play/pause, 30-second jumps and a slider to go anywhere in the film.
+Devices appear as they answer, usually within a couple
 of seconds; the search carries on for a few more, for a TV that is still
 waking up.