feat(portal): async encode pipeline - decouple capture from encoding

Split synchronous encode pipeline so sws_scale + libx264 runs on a
dedicated thread, leaving only VAAPI import + GPU scale + GPU→CPU
transfer on the main capture thread.

Problem: encode_p95 occasionally hit 74ms, blocking the entire capture
pipeline and causing capture_gap_max=356ms stutter.

Solution:
- avhw.rs: Split SwEncState into SwEncImport (main thread: VAAPI import,
  filter_graph scale, GPU→CPU transfer) and SwEncEncode (encode thread:
  sws_scale NV12→YUV420P, libx264 encode). New CpuNv12Frame struct
  carries owned pixel data across threads via crossbeam channel.
  SwEncState wraps both for backward compat (MP4/sync path untouched).
- state_portal.rs: WebRTC portal path spawns 'wl-webrtc-encode' thread
  with bounded(2) input channel (drop-newest backpressure) and separate
  timing channel. Graceful shutdown: drop webrtc_rx → drop input_tx →
  join encode thread → flush sync encoder.
- stats.rs: Add record_import() + record_encode_thread() for async timing.

Results: encode_p95 stable at 2.9-4.2ms (was 11-74ms), capture_fps
stable 59-60fps, cap_gap_p95 17-19ms. Remaining capture stalls traced
to PipeWire compositor frame delivery (external, not our code).

Cargo.lock

@@ -1126,6 +1126,15 @@ version = "0.4.29"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "5e5032e24019045c762d3c0f28f5b6b8bbf38563a65908389bf7978758920897"
 
+[[package]]
+name = "matchers"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d1525a2a28c7f4fa0fc98bb91ae755d1e2d1505079e05539e35bc876b5d65ae9"
+dependencies = [
+ "regex-automata",
+]
+
 [[package]]
 name = "memchr"
 version = "2.8.0"
@@ -2025,10 +2034,14 @@ version = "0.3.23"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cb7f578e5945fb242538965c2d0b04418d38ec25c79d160cd279bf0731c8d319"
 dependencies = [
+ "matchers",
  "nu-ansi-term",
+ "once_cell",
+ "regex-automata",
  "sharded-slab",
  "smallvec",
  "thread_local",
+ "tracing",
  "tracing-core",
  "tracing-log",
 ]