docs: highlight rule engine and Z3 improvements

[Dev-journals]

Changes

• Rule Engine Orchestration: Added 20 integration tests covering registry population, per-rule firing, run_all/run_by_name consistency, determinism, custom regex rules, and workspace-scale e2e scans
• Z3 Backend Refactor: Split monolithic smt.rs into focused sub-modules (types, invariants, backend, benchmark) with 21 integration tests
• CI: Added dedicated rule-engine-e2e and smt-module-boundaries jobs to rust.yml workflow
• Documentation: Updated DOCUMENTATION_INDEX.md with test file links, formal specs, and CI job references

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Maintenance or refactor

Testing

List the commands you ran and the scope of validation.

cargo fmt --all --check
cargo clippy --workspace --all-targets --all-features -- -D warnings
cargo test -p sanctifier-core --all-features
cargo test -p sanctifier-cli
cd frontend && npm test

Checklist

• I ran the relevant tests locally, or explained why they were not needed.
• I updated documentation for any user-facing behavior changes.
• I added or updated tests for the change when appropriate.
• I added a changelog or release-notes entry when needed, or confirmed none is required.
• I verified this branch is up to date with main and merge conflicts are resolved.

Closes #1351
Closes #1655
Closes #1656

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