feat(app): adaptive quality, frame rate capping, and tighter VBV

Three changes to make the stream survive constrained links:

Adaptive quality: the sender pipeline now tracks the PLI rate from
the receiver. Every 5 seconds it evaluates: >0.5 PLI/s means the
link is saturated (the receiver is dropping frames), so the CRF
increases by 2 (lower quality, fewer bits) and the encoder restarts
with a keyframe. <0.1 PLI/s means the link is stable, so the CRF
decreases by 1 (better quality) and the encoder probes upward.
Clamped to [user CRF, user CRF + 10] so quality never degrades
below what the link can handle, and never exceeds what the user
asked for. The adaptation is logged to stderr for visibility.

Frame rate capping (--fps N): throttles the capture loop to N
frames per second (0 = no cap; monitor rate). At 15fps instead of
60fps, the bandwidth requirement drops 4x at the same quality
level. Desktop content is still smooth at 15-20fps.

Tighter VBV: one frame period of buffer instead of two. A two-frame
buffer lets a keyframe spike to twice the target rate in one burst,
which overflows any constrained hop (Wi-Fi hotspot, slow switch)
and cascades into PLI storms. One frame period keeps bursts
within what the link can absorb in real time.

include/screencast/app/cli.h

@@ -12,6 +12,7 @@ struct SendCommand {
     std::string_view peer_address;       // optional; empty means auto-discover
     int bitrate_kbps = 4000;             // VBV max bitrate
     int crf = 22;                        // constant rate factor (quality)
+    int fps = 0;                         // 0 = no cap (capture rate)
 };
 
 struct ReceiveCommand {

include/screencast/app/pipeline.h

@@ -27,6 +27,10 @@ struct SenderPipelineConfig {
     // cannot show.
     int max_encode_width = 0;
     int max_encode_height = 0;
+    // Cap the encoding frame rate (0 = no cap; use the capture rate).
+    // Lowering the frame rate halves the bandwidth at the same quality
+    // level — useful on constrained links.
+    int max_frame_rate = 0;
 };
 
 struct ReceiverPipelineConfig {