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>

packaging/debian/build-client-deb.sh

@@ -43,6 +43,10 @@ install -Dm0644 scripts/99-punktfunk-client-net.conf \
 install -Dm0644 LICENSE-MIT                              "$DOCDIR/LICENSE-MIT"
 install -Dm0644 LICENSE-APACHE                           "$DOCDIR/LICENSE-APACHE"
 install -Dm0644 README.md                                "$DOCDIR/README.md"
+# Third-party crate attributions (regenerate with scripts/gen-third-party-notices.sh).
+if [ -f THIRD-PARTY-NOTICES.txt ]; then
+    install -Dm0644 THIRD-PARTY-NOTICES.txt "$DOCDIR/THIRD-PARTY-NOTICES.txt"
+fi
 
 cat > "$DOCDIR/copyright" <<EOF
 Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
@@ -50,7 +54,7 @@ Upstream-Name: punktfunk
 Source: https://git.unom.io/unom/punktfunk
 
 Files: *
-Copyright: punktfunk contributors
+Copyright: unom and the punktfunk contributors
 License: MIT or Apache-2.0
  Dual-licensed. Full texts in /usr/share/doc/$PKG/LICENSE-MIT and
  /usr/share/doc/$PKG/LICENSE-APACHE.