[codex] Add graph cache directory option

[DanielSeemaier]

Summary

Adds a --graph-cache-dir option to the shared-memory KaMinPar binary. When provided, KaMinPar checks whether the cache directory contains a graph file with the same basename as the -G/--graph input and uses the cached copy only if its file size matches the original input graph.

This lets benchmark invocations continue to reference graphs on a shared filesystem while opportunistically reading matching local SSD copies when they are available.

Validation

• clang-format -i apps/KaMinPar.cc
• git diff --check
• cmake -S . -B build-codex -DCMAKE_BUILD_TYPE=Debug -DINSTALL_KAMINPAR=OFF -DKAMINPAR_BUILD_WITH_CCACHE=OFF
• cmake --build build-codex --target KaMinParApp -j 8
• CLI help smoke check for --graph-cache-dir and absence of --local-scratch
• Cache-hit smoke test with a corrupted original and same-sized valid cached graph
• Cache-miss smoke test with a wrong-sized cached graph falling back to the original