Implement async Database API (breaking change)

[Copilot]

Converts all GraphDatabase public methods from synchronous to async-returning Promises, enabling browser WASM compatibility where synchronous file I/O is unavailable.

API Changes

Migration example

// Before (v0.x)
const db = new GraphDatabase('./graph.db');
const node = db.createNode('Job', { title: 'Engineer' });
const results = db.nodes('Job').where({ status: 'active' }).exec();

// After (v1.0)
const db = await GraphDatabase.create('./graph.db');
const node = await db.createNode('Job', { title: 'Engineer' });
const results = await db.nodes('Job').where({ status: 'active' }).exec();

src/core/Database.ts

• Added static async create(path, options?) factory method
• Added private _getNodeSync() helper for internal sync node lookups (avoids async chains inside CRUD methods that need to validate node existence)
• Made async: createNode, getNode, updateNode, deleteNode, createEdge, getEdge, deleteEdge, transaction, export, import, close, mergeNode, mergeEdge, createPropertyIndex, listIndexes, dropIndex
• traverse() and nodes() remain synchronous — they return builder objects with no I/O; traverse() still validates node existence via direct db.prepare() call
• transaction() callback now accepts T | Promise<T>, enabling async operations inside transactions
• Fixed pre-existing TypeScript constraint error on pattern() (Record<string, unknown> → Record<string, GraphEntity>)

src/query/NodeQuery.ts

• exec(), first(), count(), exists() → async

src/query/TraversalQuery.ts

• toArray(), toPaths(), shortestPath(), paths(), allPaths() → async
• Internal private helpers (getNode, getNeighbors) remain synchronous

Tests (11 files)

• All beforeEach/afterEach and test callbacks made async
• expect(() => ...).toThrow() → await expect(...).rejects.toThrow() where needed
• Transaction callbacks updated to async (ctx) => { ... }