Fix search_graph name_pattern= performance: regex cache, LIKE pre-filter, cheap count

[awconstable]

Fixes #254

Root cause

Three compounding bugs caused name_pattern= searches to scan every node with an expensive compiled regex, regardless of how selective the pattern was:

1. sqlite_iregexp / sqlite_regexp recompiled the regex on every row — cbm_regcomp + cbm_regfree fired once per node for the full table.
2. The count query wrapped the full SELECT (including two correlated edge-count subqueries per row) in SELECT COUNT(*) FROM (...), doubling the scan with identical per-row overhead.
3. cbm_extract_like_hints was implemented and correct but never called — the LIKE pre-filter that should cut the regex scan to only matching rows was dead code.

Changes

Fix 1 — regex cached per statement (sqlite_regexp / sqlite_iregexp)
Use sqlite3_get_auxdata / sqlite3_set_auxdata to cache the compiled cbm_regex_t for the lifetime of the statement. cbm_regcomp is now called exactly once per query, not once per row.

Fix 2 — LIKE pre-filter wired in (where_add_like_hints, search_where_basic)
Wire cbm_extract_like_hints into search_where_basic via a new where_add_like_hints helper. For .*Controller.* this prepends n.name LIKE '%Controller%'; the idx_nodes_name index satisfies the LIKE clause and only matching rows reach iregexp(). Added search_like_pool_t to manage the malloc'd LIKE strings across both statement executions. ST_SEARCH_MAX_BINDS raised 16 → 32.

Fix 3 — count query stripped of per-row edge subqueries
For the common no-degree-filter path, the count SQL is now SELECT COUNT(*) FROM nodes n WHERE <same WHERE> — no correlated edges subqueries. The degree-filter path retains the wrapped form since it needs those columns for the filter.

Benchmark

Tested on a large PHP codebase (~200K nodes):

Query	Before	After	Speedup
name_pattern=.*Controller.*	3099ms	508ms	6×
name_pattern=.*Service.*	2006ms	506ms	4×
name_pattern=.*Repository.*	2006ms	508ms	4×
name_pattern=specificFunctionName	1506ms	507ms	3×
label=Method + name_pattern=.*get.*	8509ms	509ms	17×

The ~500ms floor is cold-start I/O when spawning a fresh process against a ~500MB database. In the long-running MCP server (warm file cache) the query time is sub-millisecond.

A reusable benchmark script is included at scripts/benchmark-search-graph.sh.

Tests

All store search tests pass including store_search_pagination (offset-past-end total count), store_search_degree_filter, and the full store_extract_like_hints suite.