Summary

Phase 6 of the ecosystem-wide directory structure standardization. thread_system maintains both a new include/kcenon/thread/ + src/ structure and four legacy directories (interfaces/, utilities/, core/, implementations/) — CMakeLists.txt auto-detects which mode to use. Absorb the legacy directories into the standard layout and remove the dual-mode build logic.

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

Why

• Dual structure (new include/kcenon/thread/ plus 4 legacy directories) doubles the surface area new contributors must understand
• CMakeLists.txt auto-detection logic adds 100+ lines of branching that pure standardization would eliminate
• Downstream systems (logger, monitoring, network, pacs) all depend on thread_system — its layout deviations propagate

Scope

1. Move public headers from interfaces/, utilities/, core/, implementations/ into include/kcenon/thread/{interfaces,utils,core,impl}/ (or merge into existing subdirectories)
2. Move corresponding .cpp files into src/
3. Remove the auto-detection branching in CMakeLists.txt; commit to the new structure
4. Add forwarding headers in old paths with [[deprecated]] for one release
5. Verify all 4 downstream consumers (logger, monitoring, network, database) build unchanged
6. Reorganize tests/{unit,stress,integration_tests}/ per ecosystem standard

Child Issues

Note on EPIC body: implementations/ was already removed prior to this EPIC; remaining legacy content as of decomposition: 4 headers in utilities/include/, 4 headers in core/{base,sync}/include/, plus an empty interfaces/ shell.

Acceptance Criteria

• No public headers outside include/kcenon/thread/
• No source files outside src/
• CMakeLists.txt has single build path (no legacy detection)
• Forwarding headers [[deprecated]] with removal in next minor
• All 4 downstream consumers build with at most deprecation warnings
• cmake modules match the canonical template