Place windows at map time so stream restarts never flash

Diagnosis: individual tiles flickered when their stream died and mpv
relaunched (confirmed via status: a PID dropping to 0 and coming back).
The restarted window mapped at the video's native size wherever sway
dropped it, and the corrective loop only snapped it into its cell on the
next tick — up to 2s later on the relaxed cadence.

Fix, at the source instead of racing the map:
- Pre-install a per-slot for_window rule (matched on each mpv's unique
  window title, anchored so slot-1 never matches slot-10) so sway
  positions and sizes the window synchronously the moment it maps.
- Pass --geometry=WxH per tile so mpv opens at the tile size rather
  than resizing itself to the video's native size on load.

The placeLoop remains as a corrective backstop for mid-life drift.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/compositor/compositor.go

@@ -185,3 +185,17 @@ func (c *Client) Place(ctx context.Context, pid int, r Rect) error {
 func (c *Client) PrepareForMPV(ctx context.Context) error {
 	return c.run(ctx, `for_window [app_id="mpv"] floating enable, border none`)
 }
+
+// PlaceOnMap installs a for_window rule that positions any window with the
+// given title at rect the moment it maps. This is what keeps a restarting
+// stream from flashing at the wrong place/size: sway applies the geometry
+// synchronously at map time, before the daemon's corrective loop ever sees
+// the window. Re-installing a rule for the same title replaces it. The title
+// is anchored (^...$) so slot-1 never matches slot-10.
+func (c *Client) PlaceOnMap(ctx context.Context, title string, r Rect) error {
+	cmd := fmt.Sprintf(
+		`for_window [title="^%s$"] floating enable, border none, move absolute position %d %d, resize set %d %d`,
+		title, r.X, r.Y, r.W, r.H,
+	)
+	return c.run(ctx, cmd)
+}