chore(licensing): LGPL FFmpeg swap, third-party notices, attribution hygiene

The MIT OR Apache-2.0 SOURCE license is clean (audit found no copied copyleft); the
gaps were all binary-distribution (Layer-2). This makes the shipped artifacts honest:

- Windows host + client: bundled FFmpeg BtbN gpl-shared -> lgpl-shared (AMF/QSV/decode
  unaffected; the GPL-only x264/x265 were never used), and ship the FFmpeg LGPL notice
  + license text in the installer + MSIX (licenses/).
- THIRD-PARTY-NOTICES.txt generated + bundled into installer/MSIX/deb/rpm. Offline
  generator (scripts/gen-third-party-notices.{py,sh}) + cargo-about config (about.toml/
  .hbs) with a permissive-only accepted-license allow-list as a copyleft regression gate.
- Reword the win32u GPU-preference hook comments to reflect independent reimplementation
  (no Apollo/Sunshine GPL-3.0 source copied).
- README dual-license + inbound=outbound contributor clause + non-affiliation trademark
  disclaimer; new CONTRIBUTING.md.
- LICENSE files into the standalone driver + vk-layer workspaces; deb copyright holder
  aligned to "unom and the punktfunk contributors".

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

crates/punktfunk-host/Cargo.toml

@@ -217,6 +217,7 @@ bytemuck = { version = "1.19", features = ["derive"] }
 # nvEncodeAPI64.dll) on the linker path. Build the GPU host with `--features nvenc`.
 nvenc = ["dep:nvidia-video-codec-sdk"]
 # AMD/Intel hardware encode on Windows (AMF/QSV via ffmpeg-next). OFF by default: it needs a
-# `FFMPEG_DIR` (BtbN gpl-shared, includes `*_amf`/`*_qsv`) at build time and bundles the FFmpeg
-# DLLs at runtime. Build the all-vendor GPU host with `--features nvenc,amf-qsv`.
+# `FFMPEG_DIR` (BtbN lgpl-shared — includes `*_amf`/`*_qsv`; the GPL-only x264/x265 are never used,
+# so the LGPL build suffices and keeps the bundled DLLs LGPL, not GPL) at build time and bundles the
+# FFmpeg DLLs at runtime. Build the all-vendor GPU host with `--features nvenc,amf-qsv`.
 amf-qsv = ["dep:ffmpeg-next"]