refactor(neo4j): consolidate the graph schema into one source of truth (closes #15)

[rahlk]

The Neo4j graph contract was split across catalog.ts (node labels, relationship types + endpoints, marker labels, schema version) and schema.ts (the Cypher CONSTRAINTS/INDEXES DDL), and the constraint list hand-duplicated the (mergeLabel, key) pairs already declared in NODE_LABELS — a second place to update whenever a label was added. This consolidates the entire schema into one source-of-truth module mirroring src/schema/schema.ts: the DDL is folded in and the separate schema.ts deleted, catalog.ts is renamed to schema.ts, the uniqueness CONSTRAINTS are now derived from NODE_LABELS (one per distinct (mergeLabel, key), so a new label brings its own constraint with no second list to maintain), the curated INDEXES stay co-located, and MARKER_LABELS is exported from the barrel so consumers stop deep-importing. Behavior-preserving — same 8 constraints + 3 indexes; the only delta is cosmetic constraint naming/ordering (derived names like symbol_signature, bind variable x), and since Neo4j's IF NOT EXISTS matches on schema+type, existing databases are unaffected. Verified by typecheck, the schema-conformance suite (11 pass/5 skip/0 fail), a schema.neo4j.json regen, and a neo4j-emit smoke (graph.cypher still renders 8 constraints + 3 indexes). Git shows the move as catalog.ts deleted + schema.ts rewritten because the thin old schema.ts already occupied that path. Follow-up, out of scope: make project.ts emission schema-driven so label tuples/property names aren't restated in code. Closes #15.