fix(codec): bound keyframe bursts with VBV and microsecond time_base

The first real sender/receiver run failed permanently: the receiver
reported 'non-existing PPS 0' for every frame. Without a VBV, a
2256x1504 IDR keyframe bursts hundreds of kilobytes of back-to-back
FU-A packets, overflowing the ~208KB default UDP receive buffer; the
resulting sequence gap made the depacketizer drop whole keyframes
including their in-band SPS/PPS, so the decoder never initialized and
never recovered, because every keyframe burst overflowed again.

- Encoder: add rc_max_rate = bitrate and rc_buffer_size =
  bitrate*2/fps, capping any single frame to about two frame periods
  of bytes (~40KB at the 4Mbps default).
- Encoder: switch the time_base to microseconds. It was derived from
  the configured frame rate, which quantized capture-rate timestamps
  and duplicated pts; RTP timestamps are now lossless.
- Transport: request a 4MB SO_RCVBUF on the receive socket
  (best-effort; the kernel clamps to net.core.rmem_max).

meson test 4/4, valgrind clean (loopback + codec).

docs/RUNBOOK.md

@@ -59,8 +59,16 @@ Two terminals on the same desktop session:
 ```
 
 Expected: the receiver window shows the captured desktop in near real time.
-Stop either side with Ctrl-C. Optional flags: `--port`, `--peer HOST[:PORT]`,
-`--bitrate KBPS`, `--target window`.
+If the receiver starts after the sender, a few `non-existing PPS` decode
+errors are normal until the next keyframe arrives (the GOP is ~0.4s);
+video should appear within a second. Stop either side with Ctrl-C.
+Optional flags: `--port`, `--peer HOST[:PORT]`, `--bitrate KBPS`,
+`--target window`.
+
+Notes: the encoder runs a VBV that caps keyframe bursts to roughly two
+frame periods of bytes, and the receiver requests a 4MB UDP receive
+buffer (clamped by `net.core.rmem_max`). For very high `--bitrate`
+values, raise `net.core.rmem_max` on the receiver machine.
 
 The headless equivalent runs as part of `meson test` (`udp loopback` test):
 synthetic frames → encode → packetize → localhost UDP → depacketize →