Add selective compression mode (Pattern 2) to AliasDataFrame

Implementation:
- Add selective compression: compress_columns(spec, columns=[subset])
- Add idempotent compression (skip if same schema)
- Add schema update support for SCHEMA_ONLY/DECOMPRESSED columns
- Add enhanced validation (column existence, spec validation)
- Add _schemas_equal() helper method for schema comparison

Testing:
- Add 10 comprehensive tests for selective compression
- All 61 tests passing
- Test coverage ~95%

Reviews:
- GPT: No blocking issues, proceed to validation
- Gemini: High quality, proceed to deployment

Use case: TPC residual analysis (9.6M rows, 8 columns, 35% file reduction)

Backward compatible - no breaking changes

Author: miranov25