feat(data): implement core local-first data model, CRUD, and conflict… - #17

Description
This PR designs and implements the core offline-first storage and reconciliation layer for DocsGraph. It provides the schema migrations, data store class (LocalStore), conflict-resolution mechanics, and automated unit tests required to support clean offline operations and synchronization with the remote backend.

Key Implementation Details

1. Persistent Storage Schema (0002_data_model.sql)
   Created tables for all core entities: documents, parties, clauses, and relationships (representing the document graph).
   Created an append-only sync_ops table to track client mutations (create, update, delete) and their server synchronization/sequence state (seq).
   Added indexes on foreign keys (clauses.document_id), relationship lookups, and sequence tracking to ensure high-performance lookups.
2. LocalStore (packages/data/src/store.ts)
   Implemented a unified LocalStore data access layer interacting through the SqliteAdapter interface.
   Offline CRUD: Complete transaction-safe CRUD operations for all entities. Any local mutation automatically records a corresponding SyncOp to the log with a seq set to NULL to denote unsynced status.
   Auto-Migrations: Implemented the schema verification loop (initialize()) to detect database state and apply pending migrations sequentially.
3. Last-Write-Wins (LWW) Conflict Resolution (packages/data/src/sync/conflict-resolution.md)
   Documented and implemented a field-level Last-Write-Wins strategy.
   When applying incoming remote operations (applyRemoteOps):
   If the entity has no pending offline edits (seq IS NULL in sync_ops), remote changes are applied directly, and last_seq advances to the server's sequence number.
   If the entity has unsynced offline updates, the local modifications take precedence on a field-by-field level. Fields modified locally offline are preserved, while other non-overlapping fields are updated with remote values.
   Remote deletion is ignored if there are unsynced edits, preserving user input.
4. Verification & Testing (packages/data/src/store.test.ts)
   Introduced a lightweight, memory-efficient in-memory SQLite database simulator.
   Added comprehensive unit tests covering:
   Idempotent schema migrations and meta version setting.
   Full CRUD lifecycle for documents, parties, clauses, and relationships.
   Transaction-safe local operational logging.
   Server acknowledgment mapping (applyAck updating sequences).
   Field-level conflict resolution (verifying overlapping offline edits are preserved while non-overlapping fields are updated).