perf(encode/nvenc-linux): LN2 v1 — stream-ordered submit via NvEncSetIOCudaStreams

Bind the session's IO streams to the encode thread's high-priority copy
stream: in sync-retrieve depth-1 use the per-frame input copy and cursor
blend now enqueue with NO cuStreamSynchronize and encode_picture orders
after them on the stream. Same stream both directions, so the encode's
completion is inserted into the stream and later work (the next frame's
copy into a reused ring slot) waits for it.

Soundness gate: the fast path engages only when pending is empty (true
depth-1 usage) — every prior encode was drained by a blocking poll, and
the caller holds the frame payload across the matching poll (contract now
documented on Encoder::submit; both host loops already comply). Pipelined
callers and PUNKTFUNK_NVENC_ASYNC mode keep the blocking copies.

True zero-copy input registration (registering the worker-owned IPC
buffer directly) stays the LN2 v2 follow-up — it needs a contiguous
worker-pool NV12 layout and a registration<->IPC-mapping lifetime tie.

PUNKTFUNK_NVENC_STREAM_ORDERED=0 restores the old blocking behavior.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/pf-encode/src/enc/codec.rs

@@ -219,6 +219,11 @@ pub struct EncoderCaps {
 
 /// A hardware encoder. One per session; runs on the encode thread.
 pub trait Encoder: Send {
+    /// Submit one captured frame for encoding. Lifetime contract: the caller must keep `frame`
+    /// (and its GPU payload) alive until this frame's AU has been returned by
+    /// [`poll`](Self::poll) — a stream-ordered backend (Linux direct-NVENC's IO-stream binding)
+    /// may still be reading the payload asynchronously after `submit` returns. Both host encode
+    /// loops already hold the frame across their poll drain; new callers must do the same.
     fn submit(&mut self, frame: &CapturedFrame) -> Result<()>;
     /// [`submit`](Self::submit) with the **wire frame index** this frame's AU will carry — the
     /// number the packetizer stamps on it and the client's loss reports/RFI requests name. The