feat: Sheets 템플릿 가이드 보강(TDD)

- Code.gs에 @tool/@version 헤더, 필드 매핑 TODO, try/catch 에러 처리 추가
- README.txt에 Web App 배포/필드 매핑 가이드 추가
- 테스트 추가: sheets.template.enhance.test.ts
- 전체 테스트 그린 유지

README.md

@@ -97,13 +97,15 @@ Simple index to quickly find assets by group:
 
 ### assets.integrity.index.json
 
-- entries: array of `{ path, integrity }`
+- entries: array of `{ path, integrity, size }`
   - `path`: asset `zipPath` (e.g. `assets/abcd1234.png` or grouped `assets/images/...`)
   - `integrity`: `sha256-<base64>` (real SHA-256 of the asset bytes)
+  - `size`: byte length of the asset
 - bundleHash: `sha256-<base64>` over a deterministic join of `path` and `integrity` for all entries
 
 Usage:
 - Verify file-level integrity by recomputing SHA-256(base64) of each asset and comparing with `entries[].integrity`.
+- Verify file size matches `entries[].size`.
 - Verify bundle integrity by recomputing `bundleHash` from sorted entries; useful for quick tamper detection.
 
 ### Google Sheets template (optional)