feat(callout): callout block pipeline with config-driven endering

[sospartan]

Summary

Adds config-driven callout block support as a standalone pipeline, entirely
separate from blockquotes. Embedders supply type-to-color+icon mappings via
CalloutConfiguration, and the engine renders GFM-style callouts with
background fill, colored accent bar, and SF Symbol icons.

Problem

The previous callout implementation on local was tied to blockquote styling
— detection happened inside styleBlockquote() via regex, making the feature
hard to isolate, test, and review independently.

Solution

A dedicated BlockKind.callout in BlockParser, activated by a single static
injection point (BlockParser.calloutTypes). No method signatures change
anywhere in the parser/styler/tokenizer pipeline.

• Parser: calloutInfo() hand-scanner classifies > [!TYPE] lines before
  isBlockquote() in computeBlocks
• Token: single token per block (like codeBlock), so incremental editing
  naturally expands to the full callout range via tokenRestyleParagraphs
• Styler: styleCallout() independent of styleBlockquote(); title always
  visible, [!TYPE] marker hidden in render mode
• Renderer: plain rectangle background + left accent bar + SF Symbol icon
  (no rounded corners, no overlay title)
• Config: CalloutConfiguration with CalloutStyle(color:icon:) per type;
  .none default preserves backward compatibility

[sospartan]

@luca-chen198
Taking the previous feedback, I've completely refactored this PR with the following approach:

• Reference implementation: Studied CodeBlock and LaTex for the architecture.
• Minimal core impact: The changes no longer interfere with the blockquote implementation.
• Opt-in feature: Callouts are now optional and can be configured by developers.

I'm relieved the #87 wasn't merged, as it felt rough and poorly structured. This version is much cleaner, I think. I hope this is a step forward, and I'm definitely open to further feedback!