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>

Cargo.lock

@@ -340,24 +340,6 @@ version = "1.8.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2af50177e190e07a26ab74f8b1efbfe2ef87da2116221318cb1c2e82baf7de06"
 
-[[package]]
-name = "bindgen"
-version = "0.70.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f49d8fed880d473ea71efb9bf597651e77201bdd4893efe54c9e5d65ae04ce6f"
-dependencies = [
- "bitflags",
- "cexpr",
- "clang-sys",
- "itertools",
- "proc-macro2",
- "quote",
- "regex",
- "rustc-hash 1.1.0",
- "shlex 1.3.0",
- "syn",
-]
-
 [[package]]
 name = "bindgen"
 version = "0.72.1"
@@ -372,7 +354,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "regex",
- "rustc-hash 2.1.2",
+ "rustc-hash",
  "shlex 1.3.0",
  "syn",
 ]
@@ -849,9 +831,9 @@ dependencies = [
 
 [[package]]
 name = "ffmpeg-next"
-version = "7.1.0"
+version = "8.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "da02698288e0275e442a47fc12ca26d50daf0d48b15398ba5906f20ac2e2a9f9"
+checksum = "f7c4bd5ab1ac61f29c634df1175d350ded29cf74c3c6d4f7030431a5ae3c7d5d"
 dependencies = [
  "bitflags",
  "ffmpeg-sys-next",
@@ -860,11 +842,11 @@ dependencies = [
 
 [[package]]
 name = "ffmpeg-sys-next"
-version = "7.1.3"
+version = "8.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f9e9c75ebd4463de9d8998fb134ba26347fe5faee62fabf0a4b4d41bd500b4ad"
+checksum = "a314bc0e022a33a99567ed4bd2576bd58ffd8fcff7891c29194cfecc26a62547"
 dependencies = [
- "bindgen 0.70.1",
+ "bindgen",
  "cc",
  "libc",
  "num_cpus",
@@ -1430,7 +1412,7 @@ version = "0.9.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "901049455d2eb6decf9058235d745237952f4804bc584c5fcb41412e6adcc6e0"
 dependencies = [
- "bindgen 0.72.1",
+ "bindgen",
  "cc",
  "system-deps",
 ]
@@ -1862,7 +1844,7 @@ version = "0.9.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cb028afee0d6ca17020b090e3b8fa2d7de23305aef975c7e5192a5050246ea36"
 dependencies = [
- "bindgen 0.72.1",
+ "bindgen",
  "libspa-sys",
  "system-deps",
 ]
@@ -1994,7 +1976,7 @@ dependencies = [
  "pin-project-lite",
  "quinn-proto",
  "quinn-udp",
- "rustc-hash 2.1.2",
+ "rustc-hash",
  "rustls",
  "socket2",
  "thiserror 2.0.18",
@@ -2015,7 +1997,7 @@ dependencies = [
  "lru-slab",
  "rand 0.9.4",
  "ring",
- "rustc-hash 2.1.2",
+ "rustc-hash",
  "rustls",
  "rustls-pki-types",
  "rustls-platform-verifier",
@@ -2265,12 +2247,6 @@ dependencies = [
  "zeroize",
 ]
 
-[[package]]
-name = "rustc-hash"
-version = "1.1.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "08d43f7aa6b08d49f382cde6a7982047c3426db949b1424bc4b7ec9ae12c6ce2"
-
 [[package]]
 name = "rustc-hash"
 version = "2.1.2"