Summary

Phase 2 of the ecosystem-wide directory structure standardization. database_system currently lacks a public-header directory entirely — every .h file lives in database/ mixed with .cpp files. Introduce include/kcenon/database/, separate public headers from implementation, and split the single 600+-test unit_tests.cpp into per-module test files.

Part of kcenon/common_system#657 (master EPIC).

Why

• Largest structural deviation in the ecosystem — no other system mixes public headers with implementation
• Install rules currently require special-case copying from database/ tree; every consumer faces non-standard #include paths
• Single unit_tests.cpp defeats parallel test compilation and forces full rebuild on any change

Scope

1. Create include/kcenon/database/ with feature subdirectories (core, backends, query, orm, security, storage, async, monitoring, protocol)
2. Move all public .h files from database/ into include/kcenon/database/
3. Rename database/ → src/; keep only .cpp files plus private headers
4. Update all #include paths across the repo plus downstream consumers (pacs)
5. Split tests/unit_tests.cpp into ~10-15 per-module test files
6. Decompose 596-line CMakeLists.txt into cmake/*.cmake modules per the canonical template
7. Add forwarding headers in old paths with [[deprecated]] for one release

Child Issues

• Header path migration (include/ creation + moves)
• src/ rename + #include sweep
• unit_tests.cpp split per module
• CMakeLists.txt decomposition
• Forwarding headers with deprecation warnings
• Downstream pacs_system include path update

Acceptance Criteria

• All public headers under include/kcenon/database/
• No .h files remain in src/ (private headers explicitly marked or moved)
• tests/ contains per-module test files; CTest reports tests grouped by module
• cmake/*.cmake modules match the canonical template
• Forwarding headers warn on use; removal scheduled for next minor release
• Downstream pacs builds pass with at most deprecation warnings