docs: cleanup outdated documentation files

[eddmann]

What was changed:

• Removed STEP4_STATUS.md (outdated step-specific status)
• Removed docs/QUICK_WINS_ANALYSIS.md (failed optimization attempt)
• Removed docs/ROM_CHECK_ANALYSIS.md (superseded by test-results.md)
• Removed docs/profiling-analysis.md (expected analysis, never realized)
• Updated references in docs/STATUS.md and docs/performance.md

Why this cleanup:

• STEP4_STATUS.md: Step 4 completed long ago, info is in docs/STATUS.md
• QUICK_WINS_ANALYSIS.md: Post-mortem of failed quick wins, no longer relevant
• ROM_CHECK_ANALYSIS.md: Initial ROM analysis, superseded by comprehensive test-results.md
• profiling-analysis.md: Expected profiling hotspots that were never measured with actual data

Documentation retained:

• PLAN.md: Main development roadmap (reference)
• docs/STATUS.md: Current implementation status
• docs/test-results.md: Test ROM results
• docs/known-issues.md: Known issues tracking
• docs/performance.md: Performance tracking
• docs/optimizations.md: Optimization log
• docs/wasm-*.md, browser-usage.md: WASM documentation

Verification:

• All references to removed files updated
• No broken links remain in documentation
• Core documentation structure preserved