feat: Asahi validation tests, rootfs build fixes, shellcheck-clean scripts

- Add 16 validation tests: shellcheck (3 roles), installer_data.json
  schema (8), Python parser validation, ZIP structure (3), rootfs mount
- Fix empty SSH keys generating invalid bash (SC1073)
- Fix __dirname crash in ESM modules (use import.meta.url)
- Fix rootfs build: mkdir -p before writing, correct binary paths
- Add .gitignore for large build artifacts (.asahi-cache, *.zip)
- Bump smoke test timeout for additional static plugin registration

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -27,3 +27,7 @@ node_modules/
 # Task files
 # tasks.json
 # tasks/ 
+
+# Asahi build artifacts (large)
+bastion/.asahi-cache/
+bastion/asahi-repo/*.zip