Query the schema as a graph via MATCH on schema_table/schema_rel

[adsharma]

Fixes: #1011

Query the schema as a graph instead of only via the :schema CREATE-statement dump.

lbug>  MATCH (t:schema_table) RETURN t.name, t.type ORDER BY t.name;
┌─────────┬────────┐
│ t.name  │ t.type │
│ STRING  │ STRING │
├─────────┼────────┤
│ City    │ NODE   │
│ Follows │ REL    │
│ LivesIn │ REL    │
│ User    │ NODE   │
└─────────┴────────┘
(4 tuples)
(2 columns)
Time: 1.47ms (compiling), 1.02ms (executing)
lbug>  MATCH (s:schema_table)-[r:schema_rel]->(d:schema_table) RETURN s.name, r.name, d.name ORDER BY s.name;
┌────────┬─────────┬────────┐
│ s.name │ r.name  │ d.name │
│ STRING │ STRING  │ STRING │
├────────┼─────────┼────────┤
│ User   │ LivesIn │ City   │
│ User   │ Follows │ User   │
└────────┴─────────┴────────┘
(2 tuples)
(3 columns)
Time: 1.07ms (compiling), 1.36ms (executing)

How it works:

• New system tables schema_table (node: name PK, id, type) and schema_rel (rel: name, id) mirror the catalog. Prefixed names because bare table is a reserved keyword and MATCH (a:table) is a parse error.
• Auto-refreshed before MATCH queries via a Connection-layer hook (best-effort; main DB only).
• Hidden from SHOW_TABLES/:schema/wildcard MATCH/EXPORT (visible with internal catalog enabled for cleanup).

Tests: new test/test_files/schema/schema_graph.test; shell suite 14 passed; e2e ddl/match/comment/issue/schema suites passed.