feat(cypher): bounded EXISTS { } pattern predicate in WHERE

Add a single-hop, anchored existence predicate:
  WHERE NOT EXISTS { (f)<-[:CALLS]-() }   -- functions with no caller
  WHERE EXISTS { (f)-[:CALLS]->() }       -- functions that call something

Parsed via parse_exists_predicate (reusing parse_node/parse_rel) into a leaf
condition (op=EXISTS, anchor variable, edge type, direction); evaluated against
the bound node with cbm_store_find_edges_by_source_type / _by_target_type. This
is edge-type-specific, so it finds true orphans that in_degree/out_degree miss
(e.g. a node with only a DEFINES edge but no CALLS caller). Multi-hop / nested-
WHERE EXISTS is intentionally unsupported and errors clearly.

Tests cover the dead-code (NOT EXISTS) and has-outgoing (EXISTS) cases; smoke
coverage added. ASan-clean.

Refs: Cypher read suite (EXISTS predicate)