Code-block enrichment

Add diff markers and per-line highlighting to fenced code blocks.

The codeEnrichment feature adds two interactive code-block enrichments:

1. Diff markers — annotate added/removed lines via // [!code ++] / // [!code --] comments.
2. Line highlighting — highlight specific lines by adding a range like {1,3-5} to the fence info-string.

Both behaviors run as a hast-phase visitor after the syntect highlighter, operating on the per-line <span class="line"> structure it emits. Reference: rehype-pretty-code.

Enable

// zfb.config.ts
export default defineConfig({
  markdown: {
    features: {
      codeEnrichment: {},
    },
  },
});

Both diffMarkers and lineHighlight are on by default when the feature is enabled. To disable either independently:

codeEnrichment: {
  diffMarkers: false,   // disable diff-marker processing
  lineHighlight: true,
},

Diff markers

Add // [!code ++] or // [!code --] as a comment at the end of a line. The marker comment is stripped from the visible output. The matching <span class="line"> receives data-line-diff="added" or data-line-diff="removed".

```js
const unchanged = 1;
const removed = 2; // [!code --]
const added = 3; // [!code ++]
```

Use these data attributes in CSS to style the diff:

span.line[data-line-diff="added"]   { background: rgba(0, 255, 0, 0.1); }
span.line[data-line-diff="removed"] { background: rgba(255, 0, 0, 0.1); }

Supported comment styles: // (JS/TS/Rust), # (Python/Ruby/Shell), -- (SQL/Lua).

Line highlighting

Add a brace-delimited range after the language identifier in the fence info-string. Matching lines receive data-line-highlight="true" on their <span class="line">.

```js {1,3-5}
const a = 1;
const b = 2;
const c = 3;
const d = 4;
const e = 5;
const f = 6;
```

Lines 1, 3, 4, and 5 get data-line-highlight="true". Style them in CSS:

span.line[data-line-highlight="true"] { background: rgba(255, 255, 0, 0.1); }

The range syntax supports:

• Single numbers: {3}
• Ranges: {3-5} (inclusive)
• Combinations: {1,3-5,8}

Combining both

Diff markers and line highlighting work independently on each line, so the same line can have both:

```js {2}
const x = 1; // [!code ++]
const y = 2;
```

Line 1 gets data-line-diff="added" (diff marker). Line 2 gets data-line-highlight="true" (highlight range).