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>

bastion/package.json

@@ -24,7 +24,9 @@
     "test:integration:arm-iso": "vitest run -c tests/integration/vitest.config.ts -t 'ARM ISO'",
     "test:integration:arm-iso:host": "sudo -E $(which npx) vitest run -c tests/integration/vitest.config.ts -t 'ARM ISO'",
     "test:integration:asahi": "vitest run -c tests/integration/vitest.config.ts -t 'asahi firstboot'",
-    "test:integration:asahi:host": "sudo -E $(which npx) vitest run -c tests/integration/vitest.config.ts -t 'asahi firstboot'"
+    "test:integration:asahi:host": "sudo -E $(which npx) vitest run -c tests/integration/vitest.config.ts -t 'asahi firstboot'",
+    "test:integration:asahi-validate": "vitest run -c tests/integration/vitest.config.ts -t 'asahi.*validation'",
+    "test:integration:asahi-validate:host": "sudo -E $(which npx) vitest run -c tests/integration/vitest.config.ts -t 'asahi.*validation'"
   },
   "engines": {
     "node": ">=20.0.0",