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>

docs-site/content/docs/configuration.md

@@ -143,7 +143,8 @@ notes for context.
 | Setting | Values | Meaning |
 |---|---|---|
 | `PUNKTFUNK_GSO` | `1` · `0` | UDP Generic Segmentation Offload on the send path (coalesce a frame's packets into kernel super-buffers) — cuts send CPU ~30%, but its line-rate packet trains can cost delivered throughput on constrained links (measured on a 2.5GbE hop). Off by default until send pacing spaces the super-buffers; set `1` to opt in (auto-falls back to `sendmmsg` on kernels/paths without support). |
-| `PUNKTFUNK_SPLIT_ENCODE` | `0`/`disable` · `1`/`auto` · `2` · `3` | NVENC N-way split-encode for very high pixel rates (5K@240). `auto` picks automatically above ~1 Gpix/s. |
+| `PUNKTFUNK_SPLIT_ENCODE` | `0`/`disable` · `1`/`auto` · `2` · `3` | NVENC N-way split-encode for very high pixel rates (5K@240). `auto` picks automatically above ~1 Gpix/s. H.264 never splits (not applicable per the SDK); on HEVC a *forced* split disables sub-frame readback (mutually unsupported) — set `0` to choose sub-frame instead. |
+| `PUNKTFUNK_NVENC_SUBFRAME` | `0` · `1` | NVENC sub-frame (slice-level) readback for lower latency on sync sessions. Default: on where the GPU supports it (Linux direct NVENC). `0` = never; `1` = force. On HEVC it yields to a forced split-encode (the SDK documents the pair unsupported). |
 | `PUNKTFUNK_GPU_PRIORITY_CLASS` | `off` · `normal` · `high` · `realtime` · `auto` | **(Windows)** GPU scheduling priority for capture/encode under a GPU-saturating game. Default `auto` (starts `high`, upgrades to `realtime` when it's safe — e.g. HAGS off); `high` pins the static pre-gate behaviour; `realtime` is the strongest lever but can freeze NVENC on some setups. |
 | `PUNKTFUNK_IDD_DEPTH` | `N` (default `2`) | **(Windows)** IDD-push pipeline depth. `1` cuts latency once GPU priority is raised; higher smooths a contended GPU. |