feat(release): add cross-platform packaging

Add target-aware engine provisioning, platform package configs, and CI/release verification for macOS arm64, Windows x64, and Linux AppImage.

THIRD_PARTY_NOTICES.md

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+# Third-Party Notices
+
+Firelink distributes separate executable tools. Firelink's MIT license does not replace their licenses.
+
+Exact versions, target hashes, sources, and build descriptions are pinned in `engines.lock.json`.
+
+## aria2
+
+- Project: <https://aria2.github.io/>
+- Source: <https://github.com/aria2/aria2>
+- License: GNU General Public License version 2 or later
+
+Corresponding source for the distributed version is available from the source link and release tag listed in `engines.lock.json`. Firelink release notes must retain that source reference.
+
+Linux x64 uses a checksum-pinned musl static build produced from upstream aria2 by <https://github.com/abcfy2/aria2-static-build>. Builder source and upstream tag are recorded in `engine-sources.lock.json`.
+
+## FFmpeg
+
+- Project and source: <https://ffmpeg.org/>
+- License information: <https://ffmpeg.org/legal.html>
+
+Current macOS binary reports `--enable-gpl --enable-version3`; distribution therefore follows GNU GPL version 3 requirements. Exact build identity is recorded in `engines.lock.json`.
+
+Windows and Linux archives come from checksum-pinned BtbN FFmpeg GPL builds. Build project: <https://github.com/BtbN/FFmpeg-Builds>.
+
+## yt-dlp
+
+- Project and source: <https://github.com/yt-dlp/yt-dlp>
+- License: The Unlicense
+
+Firelink uses a self-contained PyInstaller onedir distribution. Embedded Python packages keep their own license files inside `_internal` where supplied.
+
+## Deno
+
+- Project and source: <https://github.com/denoland/deno>
+- License: MIT
+
+## OpenSSL and bundled native libraries
+
+Engine payloads may contain OpenSSL, SQLite, c-ares, libssh2, gettext/libintl, zstd, and other runtime libraries. Their copyright and license notices remain part of their source distributions and embedded package metadata.
+
+Release engineering must review each newly added target payload before adding its hashes to `engines.lock.json`. Missing provenance or license data blocks release.