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>

crates/lumen-host/Cargo.toml

@@ -36,7 +36,7 @@ rusty_enet = "0.4"
 # `open_pipe_wire_remote` is unconditional, so ashpd's own `pipewire` feature is not
 # needed — we drive PipeWire with the `pipewire` crate below.
 ashpd = { version = "0.13", features = ["screencast", "remote_desktop"] }
-ffmpeg-next = "7"
+ffmpeg-next = "8"
 libc = "0.2"
 # Must match the pipewire crate ashpd 0.13 links (libspa/pipewire-sys `links` key is
 # unique per build), i.e. 0.9 — NOT the 0.10 the setup doc mentions.

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

@@ -1,4 +1,4 @@
-//! NVENC encoder via `ffmpeg-next` (binds the system FFmpeg 7.x / libavcodec 61).
+//! NVENC encoder via `ffmpeg-next` (binds the system FFmpeg 8.x / libavcodec 62).
 //!
 //! 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