h264enc: never crash — no unwrap/expect/unreachable, encode returns Result

- `Encoder::encode` returns `Result` (it was `try_encode(..).expect(..)`, panicking on short
  planes); tests and the example unwrap explicitly.
- P/B macroblock analysis without a reference codes intra instead of `expect`/`unwrap`;
  spatial direct without a list-1 picture predicts zero motion.
- Rate control prior, DPB sliding window, DTS queue, reconstruction buffers, CAVLC writer,
  SAD rows and forward 8x8 transforms without `unwrap`/`expect`; skip-macroblock arms in
  `write_mb` debug-assert instead of `unreachable!`.
- `#![deny(clippy::unwrap_used, …)]` outside tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/h264enc/examples/h264enc_synth.rs

@@ -64,7 +64,7 @@ fn main() {
         enc.set_recon_capture(false);
     }
     for (i, f) in frames.iter().enumerate() {
-        pk.extend(enc.encode(&f.frame(), i as i64));
+        pk.extend(enc.encode(&f.frame(), i as i64).unwrap());
     }
     pk.extend(enc.flush());
     let el = t0.elapsed().as_secs_f64();