Optimize B-tree node shifts for trivially copyable values

[sukeya]

Motivation

• Improve performance of value shifting inside B-tree nodes by taking a fast path for trivially copyable value types.
• Ensure overlapping memory moves in left/right shifts are performed safely and efficiently for contiguous trivial types.

Description

• Added includes <cstring>, <memory>, and <type_traits> and branched shift_values_right / shift_values_left on std::is_trivially_copyable_v<mutable_value_type>.
• Fast path uses std::to_address(begin) + std::memmove to move contiguous regions for trivially copyable mutable_value_type while keeping the existing std::move_backward / std::move paths for non-trivial types.
• Documented in comments that std::memmove handles overlapping ranges so right/left shifts remain well-defined for the fast path.
• Registered focused benchmarks in benchmark/btree_bench.cpp to exercise trivial-ish types (int and std::pair<int,int>) for Insert/Delete on Set and Map to observe performance differences.

Testing

• Configured the project with cmake -S . -B build which completed successfully.
• Built the main targets with cmake --build build -j4 which completed successfully.
• Attempted to build the benchmark subproject with cmake -S . -B build-bench -DPLATANUS_BUILD_BENCHMARK=ON but it failed due to inability to fetch https://github.com/google/benchmark.git in this environment (network restriction), so benchmarks could not be executed here.

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Codex Task