Release: Flatpak x86_64 + aarch64 bundles, AppImage zsync auto-update, README Downloads (#22)

- packaging/linux/package.sh: embed gh-releases-zsync update information in each AppImage
  (appimagetool -u) and write the .zsync beside it; the Linux job installs zsync and checks both.
- New flatpak job (x86_64 on ubuntu-24.04, aarch64 on ubuntu-24.04-arm) repackages the Linux
  tarball with packaging/linux/flatpak-bundle.sh and ai.storyteller.wordcraft.bundle.yml; no Rust
  build, no release environment. The release job needs it.
- version, Linux and web jobs drop `environment: release` (they sign nothing), so a branch
  workflow_dispatch dry-runs every unsigned job; the release job still needs the signed jobs.
- FreeBSD release build embeds craft-fonts like the other desktop builds and ships its licences.
- Flathub manifest: same finish-args as the bundle (adds xdg-documents), builds the CLI too;
  packaging-lint checks both manifests agree.
- README: Downloads section; docs/releasing.md updated.

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.wordcraft.desktop
           sed -e 's/@VERSION@/0.0.0/' -e 's/@DATE@/2026-01-01/' packaging/linux/ai.storyteller.wordcraft.metainfo.xml.in >"$RUNNER_TEMP/ai.storyteller.wordcraft.metainfo.xml"
           appstreamcli validate --no-net --explain "$RUNNER_TEMP/ai.storyteller.wordcraft.metainfo.xml"
-          python3 -c "import yaml; m = yaml.safe_load(open('packaging/linux/flatpak/ai.storyteller.wordcraft.yml')); assert m['id'] == 'ai.storyteller.wordcraft' and m['command'] == 'wordcraft'"
+          python3 - <<'EOF'
+          import yaml
+          src = yaml.safe_load(open("packaging/linux/flatpak/ai.storyteller.wordcraft.yml"))
+          bundle = yaml.safe_load(open("packaging/linux/flatpak/ai.storyteller.wordcraft.bundle.yml"))
+          for m in (src, bundle):
+              assert m["id"] == "ai.storyteller.wordcraft" and m["command"] == "wordcraft", 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