Release: Windows arm64, Flatpak aarch64, AppImage zsync updates, README Downloads (#9)

- Windows: add arm64 (aarch64-pc-windows-msvc) to the matrix. package.ps1, sign.ps1 and the
  WiX source already handled it; nothing in the dependency tree blocks the target (clippy -D
  warnings is clean for it).
- Flatpak: x86_64 and aarch64 bundles repackage the Linux job's tarball (PhotoCraft's
  approach: same binaries and embedded fonts as the AppImage/deb/rpm, no Rust build). The
  from-source manifest stays for Flathub; packaging-lint keeps the two in sync. Both now grant
  --socket=pulseaudio so slide-show audio/video plays in the sandbox.
- AppImage: embed gh-releases-zsync update information and publish the .zsync beside it.
- Jobs that sign nothing (version, linux, flatpak, freebsd, web) leave the release environment,
  so a branch dispatch is a dry run; the draft-release job now requires every build.
- README: Downloads section.

Co-authored-by: Brandon Thomas <shared2@storyteller.ai>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/packaging-lint.yml

@@ -54,4 +54,13 @@ jobs:
           desktop-file-validate packaging/linux/ai.storyteller.deckcraft.desktop
           sed -e 's/@VERSION@/0.0.0/' -e 's/@DATE@/2026-01-01/' packaging/linux/ai.storyteller.deckcraft.metainfo.xml.in >"$RUNNER_TEMP/ai.storyteller.deckcraft.metainfo.xml"
           appstreamcli validate --no-net --explain "$RUNNER_TEMP/ai.storyteller.deckcraft.metainfo.xml"
-          python3 -c "import yaml; m = yaml.safe_load(open('packaging/linux/flatpak/ai.storyteller.deckcraft.yml')); assert m['id'] == 'ai.storyteller.deckcraft' and m['command'] == 'deckcraft'"
+          python3 - <<'EOF'
+          import yaml
+          src = yaml.safe_load(open("packaging/linux/flatpak/ai.storyteller.deckcraft.yml"))
+          bundle = yaml.safe_load(open("packaging/linux/flatpak/ai.storyteller.deckcraft.bundle.yml"))
+          for m in (src, bundle):
+              assert m["id"] == "ai.storyteller.deckcraft" and m["command"] == "deckcraft", m["id"]
+          for key in ("runtime", "runtime-version", "sdk", "finish-args"):
+              assert src[key] == bundle[key], f"Flatpak manifests disagree on {key}: {src[key]} vs {bundle[key]}"
+          print("Flatpak manifests ok")
+          EOF