polish: discovery hardening per code review (review round 2 minors)

- Fail fast with clear RuntimeError when themes/ directory is missing
- Document underscore-prefix skip rule
- Tests: broken-manifest/mismatch/no-manifest skip case, missing-dir fail-fast
- tests/test_theme_pro_v3.py: drop fourth hardcoded list, import from core

Co-Authored-By: Claude Code <noreply@anthropic.com>

docs/superpowers/plans/2026-10-22-trade-pro-theme-port.md

@@ -12,6 +12,7 @@
 - 工作目录：`C:\Users\30997\Downloads\SiteForge_Factory_V5_Web_Docker\SiteForge_Enterprise_V8_10`（下称 `$ROOT`，bash 里用正斜杠）
 - Docker 实例已运行：`http://127.0.0.1:8787`（容器 siteforge-v8-10-siteforge-1/-worker-1），admin 密码在 `.env`
 - 测试命令：`python -m pytest -q`（当前 210 项全绿，任何改动后必须保持）；JS 检查 `node --check app/static/<file>.js`
+- **宿主机快速循环**（V8_12 起）：`PYTHONUTF8=1 .venv/Scripts/python -m pytest -q`，基线 **227 passed + 2 known-fail**（test_p0_backup.py::test_sqlite_backup_and_restore_with_media、test_p0_p4_completion_v89.py::test_restore_replaces_directory_atomically_and_preserves_previous——均为 Windows sqlite 文件锁环境问题，改动前即失败）；任何新失败都是回归。权威全量验证用容器：`MSYS_NO_PATHCONV=1 docker run --rm -v "$(pwd -W):/project" -w /project python:3.12-slim sh -c "python -m pip install -q -r requirements-dev.lock && python -m pytest -q"`（容器基线 226 passed + 1 node 环境失败）
 - cms 设计源：`D:/project/cms/runtimes/astro-site-runtime/src`（tokens.css 250 行、base.css 445 行、components/*.astro、pages/[locale]/*.astro）；像素基准 `dist/hesupipe/en/`
 - 改动 Python 后需 `docker compose up -d --build` 重建容器再验证 HTTP 层（构建约 1 分钟，有缓存）
 - 契约清单已固化于 spec §2（F1-F13），本计划直接引用