Fix wrapped code block selection geometry

[917Dhj]

What changed

• Preserve each TextKit selection segment's vertical bounds when cutting the code-block background.
• Add a regression test covering selection across wrapped code lines.

Why

The current renderer expands every selection segment to the full layout-fragment height. When a fenced code block wraps, those cutouts overlap unrelated visual lines and produce vertical bars, L-shaped gaps, and broken selection backgrounds.

Impact

Wrapped code-block selections now stay aligned with their visual lines. Markdown parsing, styling, and non-code selection behavior are unchanged.

Verification

• swift test — 188 tests passed.