feat(policy): enforce architecture rules from TOML

[DivyamTalwar]

Summary

• load validated architecture rules from structorium.toml
• enforce forbidden dependencies/imports between path zones during scans
• support bounded, documented exceptions with optional expiry dates
• include policy configuration in incremental-cache invalidation

Verification

• pytest -q tests/detectors/test_architecture_policy.py tests/scan/test_incremental_cache.py: 6 passed
• ruff check on all changed Python files: passed

Stack

This PR targets #60, which targets #59. The stack is required because Structorium currently disables merge commits while this task requires merge_method=merge.