build(layer1): rename standalone output app/ -> dist/; delete both App-Router move hacks

- scripts/build/prepublish.ts: APP_DIR -> DIST_DIR; remove Step 1 and Step 2.5
  hack blocks; fix MCP esbuild outfile (app/ -> dist/); update all log messages
- scripts/build/build-next-isolated.mjs: remove legacy-app-snapshot entry from
  getTransientBuildPaths() (App-Router collision hack deleted)
- scripts/build/assembleStandalone.mjs: fix standalone package.json after copy —
  removes "type":"module" so Next.js standalone server.js (CJS) loads correctly;
  also adds .build/next/ to allowed staging prefixes so server bundles are kept
- scripts/build/pack-artifact-policy.ts: app/ -> dist/ in all PACK_ARTIFACT_* paths;
  add ".build/next/" to APP_STAGING_ALLOWED_PATH_PREFIXES (Layer 1 distDir change)
- scripts/build/validate-pack-artifact.ts: dist/ check instead of app/
- scripts/build/postinstall.mjs: all app/ paths -> dist/
- scripts/build/postinstallSupport.mjs: hasStandaloneAppBundle checks dist/server.js
- bin/cli/commands/serve.mjs: APP_DIR -> dist/
- package.json: files[] "app/" -> "dist/"
- .gitignore: remove both /app and /app/ entries (no longer needed)

Smoke: NO_APP_DIR_OK; DIST_OK; check:pack-artifact PASS; health 200 from dist/

bin/cli/commands/serve.mjs

@@ -10,7 +10,7 @@ import { isTermux } from "../../../scripts/build/postinstallSupport.mjs";
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
 const ROOT = join(__dirname, "..", "..", "..");
-const APP_DIR = join(ROOT, "app");
+const APP_DIR = join(ROOT, "dist");
 
 function parsePort(value, fallback) {
   const parsed = parseInt(String(value), 10);