Reorganize standalone test harnesses under tests/:
- tests/db/: SQLite/storage backend tests (moved from db/)
- Fix missing <ctime> include in sqlitedb.cpp
- Add stubs.cpp for attr_mod_count symbols
- Update Makefile paths for new location
- tests/libmux/: New unit test harness linking against libmux.so
- 29 tests covering mux_atol, mux_atof, mux_i64toa, safe buffer
writing, StringClone, mux_stricmp, mux_strupr/strlwr,
trim_spaces, mux_strncpy
- tests/color_ops/: Unchanged (already in tests/)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
