Add synchronous node:fs support for Durable Objects

[danlapid]

Adds Phase 1 of sync filesystem support, enabling synchronous node:fs methods inside Durable Objects via LocalDOFilesystem.

Key changes:

• Add SyncWorkerFilesystem interface with sync methods (*Sync variants)
• Create LocalDOFilesystem that operates directly on ctx.storage.sql
• Unify mount API: single mount() accepts both async and sync filesystems
• Add fs-sync.ts as drop-in replacement for node:fs (via wrangler alias)
• Async fs methods automatically fall back to sync when available
• Add 18 new tests for sync operations and async-to-sync fallback
• Add sync-do-fs example demonstrating the feature
• Update documentation with correct patterns

Usage in Durable Object:

import { DurableObject } from 'cloudflare:workers'; import { mount } from 'worker-fs-mount';
import { LocalDOFilesystem } from 'durable-object-fs'; import fs from 'node:fs';

export class MyDO extends DurableObject {
    constructor(ctx, env) {
        super(ctx, env);
        mount('/data', new LocalDOFilesystem(ctx.storage.sql));
    }
    fetch() {
        fs.writeFileSync('/data/file.txt', 'hello');
        return new Response(fs.readFileSync('/data/file.txt', 'utf8'));
    }
}

wrangler.toml alias:

[alias]
"node:fs" = "worker-fs-mount/fs-sync"