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>

crates/punktfunk-host/src/encode/linux.rs

@@ -1,4 +1,6 @@
-//! NVENC encoder via `ffmpeg-next` (binds the system FFmpeg 8.x / libavcodec 62).
+//! NVENC encoder via `ffmpeg-next` (binds the system FFmpeg — `ffmpeg-sys-next` auto-detects the
+//! installed version, so this builds against FFmpeg 7.x/libavcodec 61 *or* 8.x/libavcodec 62;
+//! validated live on Ubuntu 26.04 (FFmpeg 8) and Bazzite F43 (FFmpeg 7.1)).
 //!
 //! Input is a packed RGB/BGR CPU frame; `*_nvenc` accepts `rgb0`/`bgr0`/`rgba`/`bgra`
 //! directly and does the RGB→YUV conversion on the GPU, so the host stays off the