feat(snapshot): add --snapshot-compact for token-efficient LLM consumption

[avifenesh]

Summary

• Add --snapshot-compact boolean flag that applies four token-saving transforms to ARIA snapshots
• Link collapsing: merges link + /url: child into link "Title" -> /path
• Heading inlining: merges heading with single link child into heading [hN] "Title" -> /path
• Decorative image removal: strips img nodes with empty or single-char alt text
• Duplicate URL dedup: removes second occurrence of same URL at same depth scope
• Targets 50-70% token reduction on content-heavy pages

Implementation Details

• New compactFormat() function in scripts/web-ctl.js with hardened regex patterns ([^"]+ instead of .+ to prevent ReDoS)
• Pipeline position: trimByDepth -> compactFormat -> collapseRepeated -> textOnly -> trimByLines
• 402 tests pass (24 new tests for compactFormat + 3 edge case tests + flag parsing/validation tests)

Test Plan

• Guard clause tests (null, undefined, fallback strings, empty string)
• Link collapsing tests (simple, no /url, extra children, nested)
• Heading inlining tests (single child, multiple children, no level, plain link)
• Image removal tests (empty alt, single-char, meaningful, with children)
• URL dedup tests (same depth, different depth, scope reset)
• Edge cases (blank lines, transform interaction, dedup after collapse)
• Pipeline integration test
• Flag parsing and source validation tests
• npm test passes (402/402)
• npm run validate passes

Closes #46