feat(encode/nvenc-linux): LN1 phase-3 — multi-slice sub-frame encode DEFAULT-ON for Linux direct-NVENC

Gates recorded (nvenc-subframe-slice-output.md Phase 3): loss-harness curve
identical streamed-vs-whole; adversarial security review clean; live .21
3-leg A/B — streamed 0 corrupted frames, p99 8527→5363 µs vs sliced-whole,
first_slice_us ~500 µs of a ~1250 µs encode reaching the send thread; wire
bytes rate-governed under CBR + 1-frame VBV (the ~1-2 % slice-header cost is
absorbed by rate control, not added to the wire). GameStream: plane has zero
code diff (own RTP packetizer; shares only untouched send_pacing); a live
Moonlight re-test joins the standing owed Moonlight item.

- resolve_slices(codec, default): env 1..=32 wins (1 = the NEW explicit
  single-slice escape), else the backend default — 4 on Linux direct-NVENC,
  1 elsewhere. resolve_subframe(default_on): PUNKTFUNK_NVENC_SUBFRAME
  tri-state (0 = never, 1 = force, unset = backend default).
- Linux nvenc_cuda: resolves both ONCE in query_caps — the sub-frame default
  is gated on the GPU's SUBFRAME_READBACK cap so an unsupporting GPU never
  has enableSubFrameWrite forced into its init params — and feeds the same
  resolved values to build_config, build_init_params (open AND in-place
  reconfigure) and the chunked-poll latch.
- Windows: env-only as before (async path untouched, byte-identical config
  for unset env); LowLatencyConfig.slices + the explicit subframe param
  replace the shared env reads.
- Tests: chunked e2e now runs at the DEFAULTS (no knobs); the fallback test
  becomes the escape test (SLICES=1 disarms; SUBFRAME=0 disarms while
  4-slice encode continues on the plain poll path).

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

crates/pf-encode/src/enc/windows/nvenc.rs

@@ -37,7 +37,8 @@
 #![deny(clippy::undocumented_unsafe_blocks)]
 
 use super::nvenc_core::{
-    apply_low_latency_config, build_init_params, codec_guid, LowLatencyConfig, NvStatusExt, RFI_DPB,
+    apply_low_latency_config, build_init_params, codec_guid, resolve_slices, resolve_subframe,
+    LowLatencyConfig, NvStatusExt, RFI_DPB,
 };
 use super::nvenc_status;
 use super::{ChromaFormat, Codec, EncodedFrame, Encoder, EncoderCaps};
@@ -744,6 +745,10 @@ impl NvencD3d11Encoder {
                 av1_input_depth_minus8: if ten_bit_in { 2 } else { 0 },
                 hdr: self.hdr,
                 rfi_supported: self.rfi_supported,
+                // Env-only on Windows (default single slice): the Phase-3 default-on is a
+                // Linux-direct-NVENC decision — the Windows async path stays untouched, and a
+                // Windows operator opting in must choose slices+sync over async retrieve.
+                slices: resolve_slices(self.codec, 1),
             },
         );
         Ok(cfg)
@@ -791,6 +796,9 @@ impl NvencD3d11Encoder {
             &mut cfg,
             split_mode,
             enable_async,
+            // Windows: env opt-in only ("1"), never a default — and build_init_params
+            // additionally refuses it on an async session.
+            resolve_subframe(false),
         );
 
         match (api().initialize_encoder)(enc, &mut init).nv_ok() {
@@ -1568,6 +1576,7 @@ impl Encoder for NvencD3d11Encoder {
                     &mut cfg,
                     self.split_mode,
                     self.session_async,
+                    resolve_subframe(false),
                 ),
                 ..Default::default()
             };