Fix CBOR decoder "keyDecoder is undefined" error with runtime patch

[Copilot]

This PR fixes a critical runtime error where CBOR decoders fail with "Cannot read properties of undefined (reading 'decode')" due to a missing keyDecoder property.

Problem

Users encounter this error when using CBOR functionality:

TypeError: Cannot read properties of undefined (reading 'decode')
    at CborDecoder.key (CborDecoderBase.ts:293:33)
    at CborDecoder.readObj (CborDecoderBase.ts:253:26)

The issue occurs because this.keyDecoder is undefined in the CborDecoderBase constructor.

Root Cause

The problem stems from a re-export issue in the @jsonjoy.com/util package. The file @jsonjoy.com/util/lib/buffers/utf8/sharedCachedUtf8Decoder.js uses:

tslib_1.__exportStar(require("@jsonjoy.com/buffers/lib/utf8/sharedCachedUtf8Decoder"), exports);

However, tslib.__exportStar intentionally skips the default export. Since the original module exports the shared UTF8 decoder as default, it gets lost during re-export, making sharedCachedUtf8Decoder_1.default undefined.

Solution

This PR implements a runtime patch that:

1. Automatically detects when the default export is missing from the re-exported module
2. Imports the correct decoder directly from the source @jsonjoy.com/buffers package
3. Patches the module at runtime to restore the missing default export
4. Applies transparently when CBOR utilities are imported

Usage

The fix works automatically - no code changes required:

// This now works correctly (patch applied automatically)
import {CborDecoder} from '@jsonjoy.com/json-pack/lib/cbor/CborDecoder';

const decoder = new CborDecoder();
const encoded = new CborEncoder().encode({hello: "world"});
const decoded = decoder.decode(encoded); // ✅ Works!

CLI tools also work correctly:

echo '{"test": "data"}' | npx json-pack --cbor | npx json-unpack --cbor
# ✅ Outputs: {"test":"data"}

Testing

Added comprehensive regression tests that verify:

• CBOR decoders can be instantiated without errors
• Encoding/decoding works with string keys of various types
• Complex nested objects are handled correctly
• The runtime patch applies automatically

Backward Compatibility

The fix is completely backward compatible and requires no changes to existing code. The patch only activates when the problematic re-export is detected.

Fixes #925.

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