feat: Add spm validate command for manifest-against-HTML validation

[watashi-00]

Summary

Implement a spm validate subcommand that validates a compiled manifest.json against an HTML page snapshot, reporting which selectors matched, which binds extracted data, and which preserve slots resolved.

Motivation

The QA test suite (spm-qa-test-suite) currently performs only static documentation audits. To enable real runtime QA, the CLI needs a command that validates selector matching and data extraction against real HTML without requiring a running browser.

Proposed Usage

spm validate manifest.json --against page-snapshot.html

Expected Output

• Selector match/miss for all reconstructs and components
• Bind extraction results with extracted values
• Preserve slot resolution status
• Structured JSON with --json flag
• Exit code: 0 if all pass, 1 if failures

Implementation Notes

• Use HTML parser (libxml2, Gumbo, or lexbor) to load HTML snapshot
• Apply CSS selector matching from manifest reconstructs and components
• Execute extractor pipes (text, attr, html, hiddenInputs) against matched elements
• Output structured JSON report alongside human-readable output

Acceptance Criteria

• spm validate runs without a browser
• Reports selector match/miss for all reconstructs and components
• Reports bind extraction results with values
• Reports preserve slot resolution
• --json flag for structured output
• Exit code reflects pass/fail status