feat: add onHighlightComplete callback to Code and Diff renderables

[remorses]

Adds an onHighlightComplete callback to CodeRenderable and DiffRenderable that fires when async syntax highlighting finishes.

Use case: When capturing TUI output to generate static HTML (e.g., for web previews), syntax highlighting is async via tree-sitter. Without knowing when highlighting completes, you have to guess with arbitrary timeouts. This callback enables deterministic capture - wait for the callback before capturing output.

Example in critique (diff viewer):

// Before: guess with timeout, often miss highlighting
setTimeout(() => captureOutput(), 1000)

// After: wait for highlighting to complete
<diff 
  diff={content}
  onHighlightComplete={() => captureOutput()}
/>

For DiffRenderable with split view (2 CodeRenderables), the callback fires when both sides complete highlighting.