fix(encode/nvenc): cursor-blend PTX must carry a driver-portable ISA version

The vendored cursor_blend.ptx was regenerated with the CUDA 13.3 toolkit,
which stamps '.version 9.3' — and a driver whose JIT predates that ISA
refuses the whole module with CUDA_ERROR_UNSUPPORTED_PTX_VERSION (222),
silently killing host-side cursor compositing (on-glass: composite-mode
cursor invisible on driver 595.58, KWin leg). The kernels use only baseline
arithmetic, so hand-lower the version directive to 8.0 (CUDA 12.0), which
every supported Turing+ driver JITs. Both files now warn that an nvcc
regeneration re-stamps the version and must be re-lowered.

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

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

@@ -79,7 +79,11 @@ use nvidia_video_codec_sdk::sys::nvEncodeAPI as nv;
 /// Prebuilt PTX for the cursor-overlay blend kernels (cursor-as-metadata). Source is
 /// `cursor_blend.cu` beside this file; regenerate with
 /// `nvcc -ptx -arch=compute_75 cursor_blend.cu -o cursor_blend.ptx` after editing. JIT'd by the
-/// driver, so it runs on any Turing-or-newer GPU.
+/// driver, so it runs on any Turing-or-newer GPU. ⚠️ The `.version` stamp is load-bearing (see
+/// the comment in the .ptx): a driver refuses PTX with an ISA newer than its JIT (error 222),
+/// and a stamp older than the body's real syntax is INVALID_PTX (218) — so a new toolkit's
+/// output silently kills cursor compositing on older-driver boxes. Prefer regenerating with the
+/// OLDEST toolchain that compiles the .cu, and re-test on the oldest driver box.
 const CURSOR_PTX: &[u8] = include_bytes!("cursor_blend.ptx");
 
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