fix(encode/nvenc): cache the level ceiling, expose the applied bitrate, force 2-way split at 4K120

Three encode-time fixes from the 4K120 field analysis (sessions pinned ~107 of
120 fps, 14-15 ms reported encode):

- Ceiling truth: the codec-level bitrate clamp (binary search at session open)
  now records its result in a process-lifetime advisory cache keyed by
  GPU/config, so an ABR overshoot opens — or in-place reconfigures — straight
  AT the ceiling instead of re-running the ~6-open search (and a full rebuild
  + IDR) on every overshoot. New `Encoder::applied_bitrate_bps()` exposes the
  post-clamp rate so the session loop can stop pacing/acking a phantom
  requested rate (consumed host-side in a follow-up).

- Clamp-search hygiene: only NVENC parameter/caps rejections steer the search
  now (`NvCallError` keeps the raw status downcastable); a transient failure
  (busy engine, session limit, OOM, driver skew) propagates instead of
  shrinking into — and now caching — a bogus ceiling. The floor fallback also
  records the split mode it actually opened with, so a later reconfigure
  re-presents the real session params.

- Split-frame selection: one shared `resolve_split_mode` replaces the two
  byte-identical direct-SDK copies; the force-2-way threshold moves to
  `SPLIT_FORCE_PIXEL_RATE` (950 Mpix/s, shared with the libav path) because
  4K120 = 995,328,000 px/s missed the old `> 1e9` gate by 0.47% and stayed on
  AUTO — which never engages at 2160 px height, leaving the second NVENC
  engine idle in exactly the mode the threshold existed for. The Linux
  session-ready info! line now carries the final split mode (journals are
  INFO+; this was undiagnosable from user logs).

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

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

@@ -439,15 +439,19 @@ impl NvencEncoder {
         }
 
         // Split-frame encode across both NVENC engines (GB203 has 2) when the pixel rate exceeds
-        // a single engine's HEVC capacity (~1 Gpix/s); e.g. 5120x1440@240 = 1.77 Gpix/s needs it,
-        // @120 = 0.88 Gpix/s does not. HEVC/AV1 only (not H.264). AUTO won't engage below ~2112px
+        // a single engine's HEVC capacity; e.g. 5120x1440@240 = 1.77 Gpix/s needs it, @120
+        // (0.88 Gpix/s) does not. HEVC/AV1 only (not H.264). AUTO won't engage below ~2112px
         // height, so we force `2`; below the threshold we leave it AUTO (split costs ~2% BD-rate).
-        // Output is standard HEVC — transparent to the client. Override with PUNKTFUNK_SPLIT_ENCODE.
+        // Threshold shared with the direct-SDK selector ([`super::SPLIT_FORCE_PIXEL_RATE`] — set
+        // so 4K120 = 995.3 Mpix/s forces, which `> 1e9` famously missed by 0.47%). Output is
+        // standard HEVC — transparent to the client. Override with PUNKTFUNK_SPLIT_ENCODE.
         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),
-            None if matches!(codec, Codec::H265 | Codec::Av1) && pix_rate > 1_000_000_000 => {
+            None if matches!(codec, Codec::H265 | Codec::Av1)
+                && pix_rate >= super::SPLIT_FORCE_PIXEL_RATE =>
+            {
                 opts.set("split_encode_mode", "2");
                 tracing::info!(
                     pix_rate,