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/lexer.nim

@@ -158,6 +158,12 @@ type
     tkDfs
     tkPath
 
+    tkAutoIncrement
+    tkSequence
+    tkNextval
+    tkCurrval
+    tkUuid
+
     # Window functions
     tkOver
     tkPartition
@@ -389,6 +395,8 @@ const keywords*: Table[string, TokenKind] = {
   "first_value": tkFirstValue,
   "last_value": tkLastValue,
   "ntile": tkNtile,
+  "auto_increment": tkAutoIncrement,
+  "sequence": tkSequence,
 }.toTable
 
 proc newLexer*(input: string): Lexer =