perf(store): in-memory adjacency layer for trace_path and degree filters

On a kernel-scale index (8.5M nodes, 16.9M edges) trace_path's recursive
CTE and search_graph's per-row degree subqueries spend seconds in B-tree
seeks. graph_mem reads the edges once, in one sequential scan of the
covering UNIQUE index, into a typed CSR (node id -> out/in neighbours with
interned edge types) and answers both from RAM:

- store_bfs (shortest-path mode): BFS over the layer with the CTE's exact
  semantics (minimal hop per node, the start only via a cycle back to it,
  (hop, id) order, probe row, JOIN-skips for ids without a node row); node
  rows and edge collection still come from SQL. Trail mode is unchanged.
- cbm_store_search with min/max_degree: the two correlated COUNT(*)
  subqueries become cbm_adj_degree(); filter, order, page and total are the
  same SQL.

Layers live in a process-wide registry (the daemon closes its stores after
every request), refcounted across request threads, allocated through the
memory core (CBM_MEM_CLASS_STORE) and bounded: all layers stay under a
quarter of the budget, idle ones are evicted oldest-first, and a layer that
does not fit is not built (SQL path, never a partial layer). A layer is
keyed by db path + store generation + the rollback-journal file state (the
header change counter every rollback commit bumps), read in one read
transaction with the scan; only read-only query stores of sealed DBs
qualify. The file state matters: the cross-repo pass writes CROSS_* edges
into other projects' published DBs in place without a new generation; such
a DB is in WAL mode and keeps the SQL path until the next publish.

Kernel, macOS, same index, fresh daemon: trace_path schedule 7.5 s -> 21 ms
warm (1.23 s first call incl. a 1.2 s build), search_graph Function
min_degree 50 4.2 s -> 170 ms warm (1.35 s first). Layer: 16,890,273 edges,
237 MB. Responses byte-identical to the baseline on django, redis and the
kernel (tools, Cypher and extra BFS/degree cases).

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

Author: DeusData

Makefile.cbm

@@ -449,7 +449,7 @@ PREPROCESSOR_SRC = $(CBM_DIR)/preprocessor.cpp
 SQLITE_WRITER_SRC = $(CBM_DIR)/sqlite_writer.c
 
 # Store module (new)
-STORE_SRCS = src/store/store.c src/store/architecture_projection.c src/store/store_impact.c src/store/store_graph_digest.c src/store/store_content_digest.c
+STORE_SRCS = src/store/store.c src/store/graph_mem.c src/store/architecture_projection.c src/store/store_impact.c src/store/store_graph_digest.c src/store/store_content_digest.c
 
 # Cypher module (new)
 CYPHER_SRCS = src/cypher/cypher.c
@@ -744,6 +744,7 @@ TEST_STORE_SRCS = \
     tests/test_store_impact.c \
     tests/test_store_scope.c \
     tests/test_store_graph_digest.c \
+    tests/test_graph_mem.c \
     tests/test_store_search.c \
     tests/test_store_arch.c \
     tests/test_architecture_projection.c \