feat: SQLite OPFS driver for browser

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Summary

Adds @b9g/zen/sqlite-opfs entrypoint - a SQLite driver for browsers using OPFS persistence.

Usage

// worker.ts
import { createSQLiteOPFSDriver } from "@b9g/zen/sqlite-opfs";
import { Database } from "@b9g/zen";

const driver = await createSQLiteOPFSDriver("myapp.db");
const db = new Database(driver);
await db.open(1);

Technical Details

• Uses @sqlite.org/sqlite-wasm with opfs-sahpool VFS
• No COOP/COEP headers required
• Fastest OPFS performance (3-4x faster than alternatives)
• Must run in Web Worker
• Single connection (no multi-tab, but that's fine for most use cases)

Status

Draft - Pending browser tests via libuild multiplatform test runner.

The driver code mirrors the tested sqlite.ts driver, but the WASM integration needs real browser testing before merging.

Test plan

• libuild multiplatform test runner available
• Add Playwright/browser tests
• Manual testing via playground

🤖 Generated with Claude Code

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Blocked by bikeshaving/libuild#3