Add compile metrics validation status regression tests

[LuisMRaimundo]

Adds a Phase 12 compile_metrics validation/status contract layer.

New file:
tests/phase_12/test_compile_metrics_validation_status_contract_additional.py

Adds 24 helper-level regression tests covering:

• validate_compiled_density_workbook additional branches
• export-status and sheet-consistency checks
• canonical / diagnostic / legacy column separation
• duplicate metadata-key semantics
• malformed workbook-like inputs
• deterministic, non-mutating validation behavior
• minimal write/validate interaction

Validation:

• 24 passed for the new file
• 1129 passed, 3 skipped for the full suite
• compile_metrics.py coverage: 53.1% → 54.1%
• overall scoped coverage: 65.9% → 66.3%

Production code untouched.