docs: add semantic validation feature to CHANGELOG

Document the complete semantic validation system in the [Unreleased] section:
- Section classification enforcement (required/recommended/optional/discouraged/improper)
- Content pattern validation with regex matching
- Quality metrics checking (word/sentence counts)
- Modular validator architecture
- CLI integration with --semantic, --strict, --check-links flags
- 16 tests with 100% pass rate
- Complete documentation in SCHEMA_MANAGEMENT_GUIDE.md

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Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -35,6 +35,16 @@ See history/YYMMDD-ROADMAOTOPIC/ directories for planning information of closed
   - Registry schemas take precedence over filesystem (with fallback)
   - Full backward compatibility with existing single-file validation
 - Enhanced control panel UI with better resize handle positioning for improved user interaction
+- **Semantic Document Validation**: Complete semantic validation system for markdown documents against x-markitect schema extensions
+  - Section classification enforcement: required/recommended/optional/discouraged/improper sections validated
+  - Content pattern validation: required_patterns, forbidden_patterns, discouraged_patterns with regex matching
+  - Quality metrics checking: min_words, max_words, min_sentences validation with configurable thresholds
+  - Modular validator architecture: SectionValidator, ContentValidator with clean separation of concerns
+  - CLI integration: `--semantic/--no-semantic`, `--strict`, `--check-links` flags for validate command
+  - Comprehensive reporting: Detailed validation reports with errors/warnings, line numbers, matched text
+  - Test coverage: 16 tests for semantic validators, 100% passing
+  - Full documentation: Semantic validation guide in SCHEMA_MANAGEMENT_GUIDE.md with examples
+  - Complements existing structural AST validation for complete document compliance checking
 
 ### Changed
 - **Directory Reorganization**: