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>

CONTRIBUTING.md

@@ -0,0 +1,43 @@
+# Contributing to punktfunk
+
+Thanks for your interest in contributing!
+
+## Licensing of contributions (inbound = outbound)
+
+punktfunk is dual-licensed under **MIT OR Apache-2.0**.
+
+> Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in
+> the work by you, as defined in the Apache-2.0 license, shall be dual licensed as **MIT OR
+> Apache-2.0**, without any additional terms or conditions.
+
+By opening a pull request you agree to license your contribution under these terms. This is the
+standard Rust-ecosystem "inbound = outbound" model; it keeps the project's licensing unambiguous
+(including the Apache-2.0 §5 contributor patent grant) and any future relicensing clean. You retain
+the copyright to your contributions.
+
+### Do not paste copyleft (or otherwise incompatibly-licensed) code
+
+The single thing that could poison the permissive license is **copied source from a copyleft
+project**. Several adjacent projects (Sunshine, Apollo, Moonlight) are GPL-3.0. You may study them
+and reimplement a *technique*, protocol, or wire format — those are not copyrightable — but **never
+paste their code**, and do not translate a GPL implementation line-by-line. When a comment credits
+prior art, make clear it is an independent reimplementation, not a copy. The same applies to any
+third party's code under a license incompatible with MIT/Apache.
+
+If you add a new third-party dependency, it must be permissive (MIT / Apache-2.0 / BSD / ISC / Zlib /
+Unicode-3.0 / etc.). `about.toml` holds the accepted-license allow-list; regenerate the attribution
+file with `scripts/gen-third-party-notices.sh` when the dependency tree changes.
+
+## Before you push
+
+```sh
+cargo fmt --all --check
+cargo clippy --workspace --all-targets -- -D warnings
+cargo test  --workspace
+```
+
+Generated artifacts are checked in and CI fails on drift: `include/punktfunk_core.h` (cbindgen) and
+`api/openapi.json` (`cargo run -p punktfunk-host -- openapi`). Match the surrounding code's comment
+density and naming. Commit messages end with the `Co-Authored-By` trailer (see `git log`).
+
+See [`CLAUDE.md`](CLAUDE.md) for the full build/test/run guide and design invariants.