feat(schema): Separate Definition Schema from Record Schema

BREAKING CHANGE: None - fully backward compatible

## Summary
Implement clear separation between Definition Schema (blueprint) and
Record Schema (runtime snapshot) for compression configuration. This
resolves cycle detection issues when loading definition schemas onto
fresh data.

## Core Changes

### Schema Export (Definition vs Record)
- Definition schema (include_state=False):
  - Compression targets exported as physical columns (no expr)
  - Compressed storage columns (dy_c) NOT exported
  - No state/original_removed fields
- Record schema (include_state=True):
  - Compression targets exported as aliases with decompress expr
  - Storage columns included
  - Full state information preserved

### New API Methods
- export_definition_schema() - convenience for blueprint export
- export_record_schema() - convenience for snapshot export
- validate_schema() - comprehensive validation with modes:
  - check_data=True/False (data vs structure-only validation)
  - strict=True (no pending aliases/columns allowed)
  - allow_missing_columns, allow_pending_aliases (fine-grained control)

### Bug Fixes
- Early physical check now validates schema changes on compressed columns
- get_compression_state() infers SCHEMA_ONLY for definition schemas
- Subframe schema export respects include_state parameter

### Backward Compatibility
- Old-format schemas (compression target with expr) emit DeprecationWarning
- All existing schemas load without error
- Existing tests pass unchanged

## Test Coverage
- New: test_schema_definition_vs_record.py (31 tests)
- Extended: test_alias_dataframe.py (8 new tests for schema export)
- Total: 482+ tests passing

## Reviewers
Approved by: Gemini, GPT, Claude 2

Author: miranov25