refactor(cmake): decompose top-level CMakeLists.txt into cmake/*.cmake modules

[kcenon]

What

Decompose the 609-line top-level CMakeLists.txt into 7 focused cmake/*.cmake modules. The top-level file is reduced to 64 lines containing only cmake_minimum_required, project(), CMAKE_MODULE_PATH setup, and ordered include(...) calls.

Closes #581.
Part of #577.

Why

A 600+ line top-level CMakeLists.txt is hard to review and edit. Per-concern modules let git diff clearly show which axis of the build (e.g., install rules vs. test discovery) a PR touches, and the canonical kcenon template enforces ordering invariants (project → options → dependencies → targets → tests → install) that prevent subtle bugs.

How

• 7 new cmake/*.cmake modules created:

Module	Lines	Concern
cmake/options.cmake	73	All 18 option(...) declarations grouped by category, plus BUILD_INTEGRATION_TESTS cache fixup
cmake/compiler.cmake	111	C++ standard, output dirs, debug flags, CheckCXXSourceCompiles
cmake/dependencies.cmake	148	FindSystemDependency include + find_package(Threads), asio, OpenSSL, dependency probes
cmake/install.cmake	143	GNUInstallDirs, all install(...) rules, package config generation, D5 legacy-shim install (preserved byte-equivalent)
cmake/tests.cmake	27	enable_testing(), top-level test wiring
cmake/samples.cmake	18	Conditional add_subdirectory for samples/examples
cmake/summary.cmake	87	Build configuration summary printed after all *_FOUND flags settled

• Existing cmake/FindSystemDependency.cmake (165 lines) and cmake/database_system-config.cmake.in left unchanged.
• Top-level CMakeLists.txt (64 lines) consists of: cmake_minimum_required + project() + list(APPEND CMAKE_MODULE_PATH ...) + ordered include() calls + add_subdirectory(src) and conditional tests/samples.
• A 7th module (summary.cmake) was added beyond the plan's 6 because the build-configuration summary block (~78 lines) cannot be merged into install.cmake without exceeding the 150-line cap, and semantically it's status reporting unrelated to install rules.

What was deliberately preserved

• All ordering invariants: project() before any find_package, CMAKE_MODULE_PATH append before include(FindSystemDependency), GNUInstallDirs before any ${CMAKE_INSTALL_INCLUDEDIR} consumer, add_subdirectory(src) before install rules, summary last.
• D5 legacy-shim install block (if(NOT DATABASE_DISABLE_LEGACY_HEADERS)) byte-equivalent in install.cmake.
• All 18 option() defaults, 3 find_package calls, 6 install(...) calls, 6 add_subdirectory(...) calls.

Where

File	Status
CMakeLists.txt	Reduced from 609 to 64 lines
cmake/options.cmake	New
cmake/compiler.cmake	New
cmake/dependencies.cmake	New
cmake/install.cmake	New
cmake/tests.cmake	New
cmake/samples.cmake	New
cmake/summary.cmake	New
cmake/FindSystemDependency.cmake	Unchanged
cmake/database_system-config.cmake.in	Unchanged
src/CMakeLists.txt, tests/CMakeLists.txt	Unchanged (out of D4 scope per issue body)

Test Plan

• Local cmake/g++ unavailable in sub-agent environment; relied on CI for build verification.
• CI multi-platform builds (Ubuntu/macOS Debug+Release + integration tests) are the canonical validation that the build graph is preserved.
• Sub-agent statically verified ordering invariants and call counts since CMake configure errors would only surface at CI time.

Notes for Reviewers

• Behavior preservation guarantee: every option(...) default, find_package call, install(...) rule, and add_subdirectory(...) line was copied verbatim into the corresponding module. Counts checked: 18 options, 3 find_package, 6 install, 6 add_subdirectory.
• Issue body said "9 modules": the original D4 plan in the issue listed 6 canonical modules. Implementation uses 7 because summary.cmake couldn't merge into install.cmake without breaking the 150-line cap. The acceptance criterion ("≤ 150 lines per module") is met.
• Issue body said "19 options": actual count is 18 (verified by grep -c '^option('). Likely an off-by-one in the issue body when written. Functional content preserved.
• src/CMakeLists.txt not touched: D4 scope per issue body is top-level only. src/CMakeLists.txt (648 lines) could be a candidate for a future decomposition issue but is explicitly out of scope here.

Rollback

git revert of this single squash-merge commit fully restores the monolithic 609-line CMakeLists.txt. No build behavior change so revert is symmetric.