Pause VT parser for idle sessions to eliminate CPU waste

When no WebSocket client is connected, the session's readLoop still
processes every byte of terminal output through the go-te VT parser
(tracker.Feed), Screen.Draw grapheme segmentation, and string
allocations — even though nobody is consuming the screen state.
For programs like btop inside tmux that produce continuous full-screen
redraws, this causes sustained CPU usage and GC pressure over hours.

Fix: after a 10-second idle threshold (no client connected), skip
tracker.Feed() and only maintain the replay buffer. When a client
reconnects (UpdateConnector) or a screenshot is requested
(GetScreenSnapshot), rebuild the tracker by replaying the buffer
through a fresh VT parser instance.

Changes:
- Add idleSince atomic timestamp + MarkIdle() to Session interface
- handleOutput() skips tracker.Feed when idle > threshold
- UpdateConnector() clears idle flag and rebuilds tracker from replay
- GetScreenSnapshot() rebuilds stale tracker on-demand for screenshots
- Wire MarkIdle() call into handleWebSocket cleanup (client disconnect)
- Add TestIdleTrackerPauseAndRebuild covering the full lifecycle

webterm/server_test.go

@@ -88,6 +88,7 @@ func (b *blockingSession) SendMeta(map[string]any) bool       { return true }
 func (b *blockingSession) GetReplayBuffer() []byte            { return nil }
 func (b *blockingSession) ForceRedraw() error                 { return nil }
 func (b *blockingSession) UpdateConnector(SessionConnector)   {}
+func (b *blockingSession) MarkIdle()                          {}
 func (b *blockingSession) GetScreenSnapshot() terminalstate.Snapshot {
 	return terminalstate.Snapshot{Width: 80, Height: 24, Buffer: make([][]terminalstate.Cell, 24)}
 }