feat: add CacheSlot cached-state check

[chasingegg]

Summary

• Add CacheSlot::IsCached(cid_t) to check whether a cell is already LOADED or CACHED without triggering load, pin, or LRU touch behavior.
• Add a thread-safe ListNode helper for loaded/cached state checks.
• Cover loaded, cached, evicted, and out-of-range cid behavior in CacheSlot tests.

Test Plan

• ASAN_OPTIONS=detect_leaks=0 ./cachinglayer_test

Note: cmake --build build --target cachinglayer_test currently reconfigures and fails in this environment because OpenBLASConfig.cmake is missing; verification used the existing build artifacts after recompiling touched objects.