build: support FFmpeg 7.x and 8.x; fix RPM spec GPU link deps

punktfunk-host builds unchanged against either FFmpeg 7.x (libavcodec 61) or 8.x
(libavcodec 62) — ffmpeg-sys-next auto-detects the system version, and the host's
ffmpeg FFI only touches long-stable APIs. Confirmed by building + running live on a
Bazzite F43 box (FFmpeg 7.1.3): full gamescope capture → zero-copy dmabuf→CUDA →
NVENC H.265 at 1280x720x60, p50 ~0.96 ms. Just doc/spec accuracy, no code change:

- encode/linux.rs + CLAUDE.md: drop the "FFmpeg 8 only" claim; note 7.x/8.x both work.
- rpm spec: add the missing zero-copy GPU build deps the link actually needs —
  pkgconfig(gl) + pkgconfig(gbm) (mesa) — and document that -lcuda needs libcuda.so at
  link time (NVIDIA host, or the CUDA toolkit stub on a headless COPR/koji builder).
  Tracked for a proper fix: make the cuda/gbm/GL FFI dlopen-based like khronos-egl so
  the RPM builds on a GPU-less host.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -158,7 +158,9 @@ cargo run -rp punktfunk-client-rs -- --mode 1280x720x120 --out /tmp/a.h265 --inp
 ```
 
 Pinned crate facts: `ashpd` 0.13 + `pipewire` 0.9 (must match ashpd's) + `ffmpeg-next` 8.x
-(system FFmpeg 8 / libavcodec 62). Env knobs: `PUNKTFUNK_VIDEO_SOURCE=virtual|portal`,
+(`ffmpeg-sys-next` auto-detects the system FFmpeg, so it builds against **FFmpeg 7.x/libavcodec 61
+or 8.x/libavcodec 62** — validated live on Ubuntu 26.04 (8) and Bazzite F43 (7.1); the zero-copy
+FFI also link-needs `libGL`/`libgbm`/`libcuda` at build time). Env knobs: `PUNKTFUNK_VIDEO_SOURCE=virtual|portal`,
 `PUNKTFUNK_COMPOSITOR=kwin|gamescope|mutter`, `PUNKTFUNK_ZEROCOPY=1`, `PUNKTFUNK_GAMESCOPE_APP=...`,
 `PUNKTFUNK_INPUT_BACKEND=...`, `PUNKTFUNK_PERF=1` (per-stage timing), `PUNKTFUNK_VIDEO_DROP=N` (FEC
 test), `PUNKTFUNK_FEC_PCT=N`.