chore: remove obsolete TODO(#19) and dead _fps binding

Cleanup pass after the latency investigation concluded:

- state_portal.rs: remove TODO(#19) comment block (17 lines). The Option A
  upgrade path was tentatively documented during the #19 fix, but the user
  decided to accept current latency behavior. The TODO is now noise.

- state.rs:605: remove 'let _fps = self.args.fps as i64;' dead binding left
  over from #25 time_base change. The variable became unused when PTS formula
  switched from fps-multiplier to literal 90_000, and was renamed to _fps to
  silence the warning. Removing it entirely is cleaner.

No behavior change. Build clean (0 new warnings). All 96+96+3 tests pass.

src/state.rs

@@ -602,7 +602,6 @@ impl<S: CaptureSource> State<S> {
                     return;
                 }
             };
-        let _fps = self.args.fps as i64;
         // PTS in 90kHz media-clock ticks (WebRTC encoder time_base = 1/90000).
         // Must match Portal path's compute_capture_pts unit. See issue #25.
         let pts = (tv_sec as i64) * 90_000 + (tv_usec as i64) * 90_000 / 1_000_000;

src/state_portal.rs

@@ -176,23 +176,6 @@ impl StatePortal {
 
         match self.stage {
             PortalStage::WaitingForFormat => {
-                // TODO(#19): Currently the encoder initializes on first PipeWire frame,
-                // even in WebRTC mode before any client connects. The recurring 60fps
-                // idle waste (DMA-BUF import + VAAPI scale + clone + channel send) is
-                // eliminated by the paused gate in handle_pw_frame (see #19 fix).
-                // However, the one-time SwEncEncode::new_webrtc cost (~50ms, swscale
-                // context + x264 setup + YUV frame allocation) still occurs at startup.
-                //
-                // If this startup cost becomes user-perceptible, upgrade to "Option A":
-                //   1. Add PortalStage::AwaitingClient
-                //   2. Keep WebRtcState on main thread during AwaitingClient, pump
-                //      handle_signaling/poll_and_feed from main loop
-                //   3. On is_connected() == true, initialize encoder and move WebRtcState
-                //      into the WebRTC thread
-                //   4. Use resolution-aware conservative default bitrate (see #21)
-                //
-                // Trigger condition: user reports perceivable latency or CPU spike at
-                // startup. Until then, Option B is sufficient.
                 tracing::info!(
                     "First DMA-BUF frame: {}x{} format=0x{:08X} stride={} modifier=0x{:X}",
                     frame.width,