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/windows/punktfunk-host.iss

@@ -102,10 +102,17 @@ Name: "startservice"; Description: "Start the punktfunk host service now (also s
 Source: "{#BinDir}\punktfunk-host.exe"; DestDir: "{app}"; Flags: ignoreversion
 Source: "{#HostEnv}"; DestDir: "{app}"; Flags: ignoreversion
 Source: "{#Readme}"; DestDir: "{app}"; DestName: "README.txt"; Flags: ignoreversion
+#ifdef LicensesDir
+; License/attribution payload -> {app}\licenses: the project's MIT/Apache texts, the generated
+; THIRD-PARTY-NOTICES (permissive crate attributions), and (on an amf-qsv build) the FFmpeg LGPL
+; notice + license text. Staged by pack-host-installer.ps1.
+Source: "{#LicensesDir}\*"; DestDir: "{app}\licenses"; Flags: ignoreversion
+#endif
 #ifdef WithFfmpeg
 ; FFmpeg shared DLLs (avcodec/avutil/swscale/...) laid down next to the exe - the AMD/Intel
 ; (AMF/QSV) encode backend link-imports them, so the exe won't start without them. NVENC/software-
-; only builds simply omit this block.
+; only builds simply omit this block. These are unmodified BtbN *lgpl-shared* builds, linked
+; dynamically (replaceable DLLs) - FFmpeg is used under the LGPL v2.1+; see {app}\licenses.
 Source: "{#FfmpegBin}\*.dll"; DestDir: "{app}"; Flags: ignoreversion
 #endif
 #ifdef WithWeb