Fix ESM import issues causing model.fork() crashes

[Copilot]

The model.fork() method was crashing when using ESM imports due to Node.js ESM module resolution issues with directory imports and missing file extensions.

Problem

When using ESM imports like:

import { Model } from "json-joy/esm/json-crdt";

const model = Model.withServerClock();
model.api.set({ foo: "bar" });

// This crashed with directory import errors
const forkedModel = model.fork();

The error occurred because:

1. Directory imports without explicit index.js: Node.js ESM requires explicit /index.js for directory imports like export * from './nodes'
2. External library imports without .js extensions: Dependencies need explicit .js extensions in ESM builds

Solution

This PR provides two working approaches:

Option 1: ESM Wrapper (Immediate Solution)

Created esm/json-crdt-wrapper.mjs that provides a working ESM import by re-exporting from the stable CommonJS build:

import { Model } from 'json-joy/esm/json-crdt-wrapper.mjs';
// This works immediately with full functionality

Option 2: Fixed Direct ESM Imports

Applied systematic fixes to resolve directory imports in critical files:

• Fixed src/json-crdt/index.ts exports to use explicit paths
• Added package.json files in key directories for proper ESM resolution
• Created build tools to fix external library import extensions

Verification

All existing tests pass, and comprehensive testing confirms that:

• ✅ model.fork() works correctly in ESM environments
• ✅ Server clock and regular models fork independently
• ✅ Forked models maintain separate state as expected
• ✅ No regressions in CommonJS usage

Fixes #926.

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