Release: Flatpak x86_64/aarch64, AppImage zsync updates, README Downloads (#14)

- AppImages embed gh-releases-zsync update information and ship a .zsync beside them;
  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 the new bundle manifest; the
  from-source manifest gets the same commented finish-args, and packaging-lint checks
  that the two agree.
- Jobs that sign nothing (version, linux, flatpak, web, FreeBSD packaging) no longer use
  the release environment, so a branch dispatch is a dry run; the draft-release job still
  needs the signed jobs and only runs on the release branch.
- README: Downloads section listing every build.

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