Fix fork() method crash in ESM environments due to directory imports

[Copilot]

The Model.fork() method was crashing in ESM environments due to directory imports in the binary codec that don't work with Node.js ES module resolution. This affected users trying to use the library with modern ESM setups.

Problem

The issue occurred when calling model.fork() which internally uses Model.fromBinary() and Model.toBinary(). These methods depend on the binary codec (Encoder/Decoder) that used directory imports like:

import * as nodes from '../../../nodes';

Node.js ESM requires explicit file extensions and doesn't support directory imports without explicit index files, causing the following error:

Error [ERR_UNSUPPORTED_DIR_IMPORT]: Directory import '/path/to/nodes' is not supported resolving ES modules

Solution

1. Fixed source code: Changed directory imports to explicit named imports in the core binary codec files:

   // Before
   import * as nodes from '../../../nodes';
   
   // After  
   import {
     ConNode,
     ValNode,
     RootNode,
     // ... other specific imports
   } from '../../../nodes';

2. Enhanced build process: Added an automated ESM import fixer script that:

   • Adds .js extensions to relative imports
   • Converts directory imports to explicit index.js imports
   • Handles edge cases in the compiled ESM output

3. Updated build script: The build:esm command now automatically applies ESM fixes post-compilation

Usage

The fix enables seamless ESM usage:

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

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

// This now works correctly ✅
const forkedModel = model.fork();

// Binary serialization also works ✅
const binary = model.toBinary();
const restored = Model.fromBinary(binary);

Testing

• All existing tests pass with no regressions
• Fork functionality verified with both server and logical clocks
• Binary serialization/deserialization confirmed working
• Multiple fork scenarios tested successfully

Fixes #926.

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