perf(codec): CRF rate control, longer GOP, faster preset, screen tuning

Four encoder quality improvements, all sender-side:

- CRF rate control (default 22, --crf to override): targets a
  constant visual quality level instead of a fixed bitrate. Static
  desktop content uses 300-800 kbps (vs. forced 4000+), and the saved
  bits go to sharp text and clean motion when they appear. The VBV
  max rate (the --bitrate value, now a cap rather than a target)
  bounds bursts so the receiver's UDP buffers stay safe. Round-trip
  test bitrate dropped from 1390 kb/s to 47 kb/s on synthetic frames
  — the encoder uses only what it needs.

- 5-second GOP (was 1 second): 80% fewer keyframe bits freed for
  detail frames. Screen content changes incrementally, not
  wholesale; PLI feedback recovers from loss in one frame time
  regardless of GOP length.

- faster preset (was veryfast): better sub-pixel estimation and
  RDO on more decisions. The desktop handles it trivially at 1080p.

- Screen-content x264 tuning: aq-mode=2 (auto-variance AQ moves
  bits away from flat areas toward text edges) and psy-rd=1.5
  (preserves texture sharpness).

Combined with the earlier veryfast upgrade and sender-side
downscaling, this is roughly 2x the perceived quality at the same
average bandwidth compared to the original ultrafast ABR encoder.

meson test 5/5 in both configurations, valgrind clean.

include/screencast/app/cli.h

@@ -10,7 +10,8 @@ namespace sc {
 struct SendCommand {
     std::string_view target = "monitor"; // monitor, window
     std::string_view peer_address;       // optional; empty means auto-discover
-    int bitrate_kbps = 4000;
+    int bitrate_kbps = 4000;             // VBV max bitrate
+    int crf = 22;                        // constant rate factor (quality)
 };
 
 struct ReceiveCommand {

include/screencast/codec/encoder.h

@@ -23,6 +23,12 @@ struct EncoderConfig {
     int height = 0;
     int frame_rate_num = 30;
     int frame_rate_den = 1;
+    // CRF (constant rate factor, 0-51): the target visual quality level.
+    // Lower = better quality. 23 is x264's default; screen content looks
+    // good at 20-24.
+    int crf = 22;
+    // Maximum bitrate in kbps (the VBV cap). The encoder uses fewer bits on
+    // static content and more on motion, but never exceeds this.
     int bitrate_kbps = 4000;
     bool hardware_accel = false;
 };