feat: add ID generators, fix 6 forum-discovered deficiencies

ID Generators:
- AUTO_INCREMENT for INTEGER columns
- SERIAL / BIGSERIAL as syntactic sugar
- gen_random_uuid() / uuid() for UUID v4 generation
- nextval() / currval() for sequence support
- snowflake_id(node_id) for distributed 64-bit IDs
- RETURNING clause for INSERT

Deficiency Fixes:
- Comma join: FROM t1, t2 now works as implicit CROSS JOIN
- DEFAULT in restoreSchema: defaults survive server restart
- Duplicate column names: t.id instead of id in joins
- Empty result sets: always send column metadata to client
- GROUP BY non-agg columns: return first row value instead of empty
- Aggregate column names: count(*) instead of count()

Bug fix: PK uniqueness check now uses correct key format

src/barabadb/query/ast.nim

@@ -250,6 +250,7 @@ type
       cdName*: string
       cdType*: string
       cdConstraints*: seq[Node]
+      cdAutoIncrement*: bool
     of nkConstraintDef:
       cstName*: string
       cstType*: string