fix(main): call fps_limit.flush() before encoder flush on shutdown

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

.sisyphus/evidence/task-2-cargo-test.txt

@@ -0,0 +1,12 @@
+running 30 tests
+test fps_limit::tests::first_frame_is_buffered ... ok
+test fps_limit::tests::flush_returns_last_buffered ... ok
+test fps_limit::tests::frames_far_enough_output_old ... ok
+test fps_limit::tests::frames_too_close_drops_old ... ok
+test result: ok. 30 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
+
+running 4 tests
+test test_rejects_hevc_codec ... ok
+test test_rejects_invalid_args ... ok
+test test_help_flag ... ok
+test result: ok. 3 passed; 0 failed; 1 ignored