Code block enhancements: filename titles and line highlighting

[engineervix]

Summary

Two related improvements to fenced code blocks that authors commonly expect from modern doc tools:

1. Filename/title annotation — ```go title="main.go" renders a filename label above the block
2. Line highlighting — ```go {3,5-8} highlights specific lines to draw attention

Proposed behaviour

Filename title

```go title="cmd/root.go"
package main
```

Renders a label bar above the code block showing the filename.

Line highlighting

```go {2,4-6}
line 1
line 2  ← highlighted
line 3
line 4  ← highlighted
line 5  ← highlighted
line 6  ← highlighted
```

Implementation notes

• Both are implemented as a custom goldmark code block renderer (extending the existing Chroma integration in internal/parser/highlight.go)
• Parse extra attributes from the opening fence info string
• Title: inject a <div class="code-title"> before the <pre> block
• Line highlighting: wrap highlighted lines in <span class="highlight-line">, style with CSS
• No new dependencies needed