fix: first code comment line dropped in code blocks (5 en pages affected)

[jp-knj]

Problem

First comment line in code blocks (typically filename comments like // src/components/File.astro) is dropped.

Affected: ~5 en pages × 14 locales ≈ 70 pages total

Example

markflow: "---\n// Usage: <GreetingHeadline"
baseline: "---\n// src/components/GreetingHeadline.astro\n// Usage:"

Sample files: en/basics/astro-components, en/basics/layouts, en/guides/data-fetching

Root Cause

First line of code block content is being swallowed during rendering.