render: fix 4K playback memory growth and decode-to-target-size

Root causes found for the 4K stalls and the second-footage memory
blowup (audit + code review):

- ticket bookkeeping leaked unbounded: the procpool ticket table and
  the arena slot map only ever grew (50-100 tickets/sec during
  playback, each pinning montage params and shm region views).
  Completed/cancelled/superseded/crashed entries are now removed, and
  the arena reaps fire-and-forget tickets once finished; the sync poll
  path reaps via a terminal result() read. InFlight duplicate submits
  now answer State immediately instead of sitting in the map forever.
- decode ran a full-resolution swscale to F32 RGBA (~132 MB at 4K)
  plus a second full-res copy before downscaling to the 480px proxy:
  RetrieveVideoParams.target_size lets swscale convert AND resize in
  one pass (bilinear, matching the old Rust resampler), so a 4K
  preview frame costs ~1 MB instead of ~260 MB of churn. This applies
  to proxy AND full-res requests alike.
- per-process decoder cache was unbounded (each session pins an FFmpeg
  context + 2 native decoded frames): LRU-capped at 16, eviction drops
  the map entry (in-flight renders keep their Arc; Drop releases
  FFmpeg).
- playback window completions were not generation-gated: a stale
  render from before an edit landed in the rebuilt window (wrong frame
  displayed, fresh request blocked). Stale completions now return
  their shm slot credit instead.
- async audio prefetch used the polling ticket submit without ever
  polling: switched to the fire-and-forget submit so entries reap.

crates/oak-codec/src/decoder.rs

@@ -136,6 +136,13 @@ pub struct RetrieveVideoParams {
 	pub mode: RenderMode,
 	/// Frame alpha channel is premultiplied.
 	pub alpha_is_premultiplied: bool,
+	/// Target output size for the scaled frame: `Some((w, h))` lets the
+	/// decoder's swscale pass convert AND resize in one step (native
+	/// yuv → RGBA/F32 at the target size), skipping the full-resolution
+	/// float intermediate (a 4K frame is ~132 MB as F32 RGBA — decoding
+	/// to a 480px preview through it costs ~260 MB of churn per frame).
+	/// `None` keeps the native size (the old behavior).
+	pub target_size: Option<(u32, u32)>,
 }
 
 /// `Decoder::RetrieveAudioStatus` — outcome of an audio retrieve.
@@ -708,6 +715,7 @@ mod tests_unimplemented {
 			image_sequence_number: 0,
 			mode: RenderMode::Offline,
 			alpha_is_premultiplied: false,
+			target_size: None,
 		};
 		assert!(d.retrieve_video_frame(&p).is_err());
 		assert!(d.retrieve_video(&p).is_err());