refactor: extract WasmDatabaseEngine and platform fs helper

Splits src/core/sqlite-db.ts (1270 lines) into three modules:

- src/core/engine/wasm/WasmDatabaseEngine.ts — the engine class plus
  its sql.js type interfaces. Preserves SAVEPOINT-based updateCellBatch,
  hasJsonPatch construction-time probe, safeRollback helper, and the
  BEGIN/COMMIT path in deleteColumns/insertRowBatch byte-faithfully.
- src/core/platform/fs.ts — getNodeFs helper, lifted from sqlite-db so
  both sqlite-db and the engine can import it without re-establishing
  a circular dependency (sqlite-db → engine → sqlite-db).
- src/core/sqlite-db.ts — slim factory (createDatabaseEngine) and
  worker endpoint (createWorkerEndpoint), with WasmDatabaseEngine and
  getNodeFs re-exported for back-compat with existing imports.

Net: sqlite-db.ts 1270 → 259 lines. No public-API changes, no
behavior changes. Tests: 247/247 pass. Build: clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: zknpr