feat: Add spm apply command to apply manifest transformations to HTML snapshots

[watashi-00]

Summary

Implement spm apply manifest.json --input page-snapshot.html --output result.html that applies all manifest transformations (hide, replace, reconstruct) to an HTML snapshot and outputs the resulting HTML.

Motivation

Enable visual inspection and automated DOM comparison for QA testing. Output HTML can be opened in browser, diffed against expected output, or inspected by automated tools.

Implementation Notes

• Apply hide by removing elements
• Apply replace by substituting with component mount points
• Apply reconstruct by replacing container content with component skeleton
• Inject theme CSS variables into output head
• Extract and place preserve slots

Acceptance Criteria

• Produces valid HTML output
• Hidden elements removed
• Reconstructed sections show component placeholders with extracted data
• Theme CSS injected
• Preserve slots correctly placed