feat: add OPFSWriteAheadVFS storage wrapper

[Copilot]

Adds a new storage backend wrapping OPFSWriteAheadVFS — a synchronous OPFS VFS with WAL2-style journaling that enables concurrent reads, faster writes, and no blocking checkpoints.

Changes

• src/vfs/opfs-write-ahead.ts — useOpfsWriteAheadStorage(path, options) wrapping OPFSWriteAheadVFS; uses sync wa-sqlite.wasm build; accepts nTmpFiles, autoCheckpoint, backstopInterval
• src/types/index.ts — OPFSWriteAheadVFSOptions interface
• src/types/wa-sqlite.d.ts — module declaration for wa-sqlite/src/examples/OPFSWriteAheadVFS.js
• src/utils.ts — new isOpfsReadWriteUnsafeSupported() utility to detect OPFS readwrite-unsafe locking mode support (Chromium only)
• tsdown.config.ts / package.json — new opfs-wa build entry and ./opfs-wa package export
• .gitignore — exclude package-lock.json
• README.md — usage section with caniuse link for minimal browser version, using isOpfsReadWriteUnsafeSupported in example, Chromium-only requirement, Worker constraint, and WAL restrictions

Usage

import { initSQLite, isOpfsReadWriteUnsafeSupported } from '@subframe7536/sqlite-wasm'
import { useOpfsWriteAheadStorage } from '@subframe7536/sqlite-wasm/opfs-wa'

// must run in web worker
if (await isOpfsReadWriteUnsafeSupported()) {
  const { run, changes, lastInsertRowId, close } = await initSQLite(
    useOpfsWriteAheadStorage('test.db'),
  )
}

Restrictions: Chromium only (requires OPFS readwrite-unsafe); must run in a Worker; multi-statement write transactions require BEGIN IMMEDIATE or BEGIN EXCLUSIVE.

[subframe7536]

exclude package-lock.json; add OPFS readwrite-unsafe support check util, note the minimal version and update relative example in readme

[Copilot]

exclude package-lock.json; add OPFS readwrite-unsafe support check util, note the minimal version and update relative example in readme

Done in 3e813ab:

• package-lock.json added to .gitignore and removed from tracking
• Added isOpfsReadWriteUnsafeSupported() to src/utils.ts — detects readwrite-unsafe mode support by attempting createSyncAccessHandle({ mode: 'readwrite-unsafe' }) in the current Worker context (must run in worker, same as the VFS itself)
• README OPFS Write-Ahead section now uses isOpfsReadWriteUnsafeSupported in the example and links to caniuse for minimal browser version