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;