perf: process API Sandpack files concurrently

[humanbydefinition]

Summary

• Read and transform independent API Markdown files concurrently.
• Preserve source order when aggregating changed files, validation results, skipped counts, and skipped reasons.
• Preserve write mode behavior while allowing independent file writes to proceed without blocking one another.

Why

The API Sandpack transformer processes hundreds of independent Markdown files. Awaiting each file read and transformation in sequence extends the build preparation path unnecessarily. Processing the independent files with Promise.all improves utilization while retaining deterministic result ordering.

Scope

This PR contains only the source change described above. It does not add tests or alter generated documentation.

Validation

• git diff --check
• node --check scripts/api-sandpack-examples.mjs
• node scripts/api-sandpack-examples.mjs --check
• ./node_modules/.bin/vitepress build

The API Sandpack check reported 0 transformed files and 661 live examples across 757 scanned files. The VitePress build completed successfully. The build also reported the existing environment warning that the editor sketch loader could not fetch api.github.com.