feat(core): add onError callback to createSpecStreamCompiler

[nil957]

Summary

Adds optional onError callback to createSpecStreamCompiler for debugging malformed JSON patches during streaming.

Problem

Currently, when createSpecStreamCompiler encounters malformed JSON or patch application failures, it silently ignores them. This makes debugging difficult in production when LLMs generate invalid patches.

Solution

New Options Interface

interface SpecStreamCompilerOptions<T> {
  initial?: Partial<T>;
  onError?: (line: string, error?: unknown) => void;
}

createSpecStreamCompiler({ onError: (line) => console.warn(line) })

When onError is Called

1. Parse failures: Line starts with { but parseSpecStreamLine returns null
2. Application failures: Patch parsed successfully but applySpecStreamPatch throws (e.g., test operation mismatch)

Example Usage

const compiler = createSpecStreamCompiler<Spec>({
  onError: (line, error) => {
    logger.warn('Invalid patch line:', line);
    if (error) {
      logger.error('Patch application failed:', error);
    }
  },
});

Backward Compatibility

Old signature still works:

// Still valid
createSpecStreamCompiler({ root: '', elements: {} })

New signature:

// With options
createSpecStreamCompiler({ 
  initial: { root: '', elements: {} },
  onError: (line) => console.warn(line)
})

Use Cases

• Production error monitoring
• Development debugging
• Analytics (tracking LLM patch quality)
• Auto-repair triggers

Checklist

• Tests added for error callback scenarios
• Backward compatible
• No breaking changes
• Error callback is optional

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[vercel]

Breaking API change in createSpecStreamCompiler signature causes type errors in 3 example files that still pass Partial<T> instead of SpecStreamCompilerOptions<T>.