stems: multitrack PipeWire recorder

Records every source feeding the default sink into sample-aligned float WAV
stems plus the sink mix, so sessions can be mixed and mastered later.
Release flow mirrors galdr: tag -> build tarballs on both arches -> Gitea
release -> publish to the uhhm Arch registry (no AUR package).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

release.toml

@@ -0,0 +1,12 @@
+publish = false
+allow-branch = ["main"]
+
+[[pre-release-replacements]]
+file = "aur/PKGBUILD"
+search = "pkgver=.*"
+replace = "pkgver={{version}}"
+
+[[pre-release-replacements]]
+file = "aur/PKGBUILD"
+search = "pkgrel=.*"
+replace = "pkgrel=1"