fix(pf-encode): arbitrate NVENC split-encode vs sub-frame readback (Phase 8)

Verified against nvEncodeAPI.h's own splitEncodeMode doc (user-prompted
— the audit's 'exclusive for HEVC' one-liner deserved checking):
- H.264: split 'is not applicable' — hard-DISABLE the mode so the
  written config, CeilingKey, the split diagnostic log and the
  rejection-retry stay truthful (the retry used to re-open a
  byte-identical session after an H.264 'split rejection'). The libav
  path's operator arm gains the codec gate its auto arm always had.
- HEVC: split 'not supported if … subframe mode' — when WE force split
  (TWO/THREE/AUTO_FORCED, the 4K120 throughput lever), sub-frame yields
  with a logged escape (PUNKTFUNK_SPLIT_ENCODE=0 chooses sub-frame).
  ⚠ Keyed on FORCED modes only, never != DISABLE: AUTO(0) is the
  resolver's fallthrough for every sub-950Mpix session, and the wider
  key would have disarmed the Phase-3 chunked-poll feature fleet-wide
  (critic catch). Under AUTO the driver arbitrates — the shipped state.
- AV1: untouched — per-tile sub-frame + split are legal together.

The arbitration is a pure nvenc_core fn called by each backend BEFORE
the ladder, the ceiling key and the chunked-poll latch — all three see
the post-arbitration truth. A drop inside build_init_params would have
left poll_chunk busy-polling its whole budget every AU (numSlices stays
0 without reportSliceOffsets; both loop exits dead — critic catch).
Linux latches subframe_forced beside subframe_on at query_caps (no env
re-reads after open); Windows records the arbitrated state so
reconfigure presents exactly the params the open had (also closes the
pre-existing mid-session env-flip hazard there).

Truth-table tests in nvenc_core; PUNKTFUNK_NVENC_SUBFRAME documented
(it never was); PUNKTFUNK_SPLIT_ENCODE row updated.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/pf-encode/src/enc/linux/mod.rs

@@ -448,7 +448,17 @@ impl NvencEncoder {
         let pix_rate = width as u64 * height as u64 * fps as u64;
         let split = std::env::var("PUNKTFUNK_SPLIT_ENCODE").ok();
         match split.as_deref() {
-            Some(mode) => opts.set("split_encode_mode", mode),
+            // The operator arm gains the codec gate the auto arm always had (Phase 8): split
+            // "is not applicable to H264" per nvEncodeAPI.h, and h264_nvenc has no such AVOption
+            // — setting it would fail the open on a leftover dict entry.
+            Some(mode) if matches!(codec, Codec::H265 | Codec::Av1) => {
+                opts.set("split_encode_mode", mode)
+            }
+            Some(_) => tracing::warn!(
+                codec = codec.nvenc_name(),
+                "PUNKTFUNK_SPLIT_ENCODE ignored — split encoding is not applicable to H.264 \
+                 (nvEncodeAPI.h)"
+            ),
             None if matches!(codec, Codec::H265 | Codec::Av1)
                 && pix_rate >= super::SPLIT_FORCE_PIXEL_RATE =>
             {