perf(encode/nvenc-linux): LN3 — pipelined-retrieve escalation replaces the depth-1 async foot-gun

PUNKTFUNK_NVENC_ASYNC gains a real tri-state: 1 = always (as before, now
documented as ~+1 tick at depth-1), 0 = never (vetoes escalation), unset =
ADAPTIVE — off until the session loop's cadence-overrun detector escalates.

The host loop's adaptive-depth leaky bucket grows a second stage: once the
capturer's depth is maxed (Linux portal is permanently depth-1), it asks
the encoder for pipelined retrieve via the new Encoder::set_pipelined hook
(asked exactly once; default impl declines, Windows untouched).

nvenc_cuda engages at a safe point via a clean session rebuild WITHOUT the
IO-stream binding: with input==output stream bound, later stream work
waits on prior encode completions and would serialize a pipelined session
— stream-ordered submit and two-thread retrieve are mutually exclusive.
The ordered gate now also requires async_rt absence (belt-and-braces for
the runtime switch). Re-open's first frame is the standard session IDR.

On-hardware test: escalate mid-session → retrieve thread live, binding
gone, all AUs deliver, first post-escalation AU is the IDR.

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

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

@@ -269,6 +269,15 @@ pub trait Encoder: Send {
     fn invalidate_ref_frames(&mut self, _first_frame: i64, _last_frame: i64) -> bool {
         false
     }
+    /// Escalate into a pipelined (two-thread) retrieve mode under sustained GPU contention — the
+    /// encoder analog of the capturer depth escalation: AUs ride ~one loop tick behind (`poll`
+    /// may return `None` while an encode is in flight) in exchange for capture/submit no longer
+    /// serializing on the encode wait. Returns whether pipelined retrieve is (now) active; the
+    /// switch may be deferred to the next safe point internally. `false` from the default impl =
+    /// unsupported — the session loop stops asking. De-escalation is a v2 item everywhere.
+    fn set_pipelined(&mut self, _on: bool) -> bool {
+        false
+    }
     /// Pull the next encoded AU if one is ready.
     fn poll(&mut self) -> Result<Option<EncodedFrame>>;
     /// Tear the underlying hardware encoder down and rebuild it in place, keeping the session's