build: migrate to ffmpeg-next 8 (FFmpeg 8.x / libavcodec 62)

Ubuntu 26.04 ships FFmpeg 8.0 (libavcodec 62); bump ffmpeg-next 7.1 -> 8.1 to bind it
as the intended pairing. No source changes needed — the encode API surface we use
(avcodec_send_frame, hwframe contexts, AV_PIX_FMT_CUDA, av_log) is stable across 7->8.
Workspace builds + all tests green; clippy/fmt clean. Refresh the 7.x doc references.

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

docs/linux-setup.md

@@ -130,8 +130,8 @@ Crate choices, verified current:
   `AvailableSourceTypes` is `1` (Monitor only); asking for `Window`/`Virtual` invalidates
   the session. Set `XDG_CURRENT_DESKTOP=sway` so the wlr portal backend is chosen, and
   import it into the portal's environment (see "Portal bring-up" below).
-- **Encode:** [`ffmpeg-next`](https://crates.io/crates/ffmpeg-next) **7.x** (binds the
-  system FFmpeg 6.1.1 via pkg-config; needs `clang`/`libclang`). Select the encoder by
+- **Encode:** [`ffmpeg-next`](https://crates.io/crates/ffmpeg-next) **8.x** (binds the
+  system FFmpeg 8.x via pkg-config; needs `clang`/`libclang`). Select the encoder by
   name — `encoder::find_by_name("hevc_nvenc")`, *not* by codec id (that's the SW encoder).
   Low-latency opts: `preset=p1`, `tune=ull`, `rc=cbr`, `bf=0`, `delay=0`, large `g`.
   If your FFmpeg is in a non-standard prefix, `export FFMPEG_DIR=/that/prefix`.