fix(graph): route the new scratch through the memory core

The lint chain stops at its first red target, so a clang-format failure hid the memory-core linter
until PR CI: the coverage line-offset table, the Cypher direction swap and the search label each
added a raw allocator site. The table now comes from cbm_alloc/cbm_free (extract class), the swap
frees through safe_str_free, and the BM25 row array moves to cbm_calloc/cbm_free so mcp.c's raw-site
count goes down (baseline 814 -> 813) rather than up.

Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>