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/core/server.nim

@@ -478,9 +478,8 @@ proc handleClient(server: Server, client: AsyncSocket, clientId: int) {.async.}
             slowQueryLog(slowLog, queryStr, durationMs, clientId)
 
           if success:
-            if result.rows.len > 0:
-              let dataMsg = serializeResult(result, header.requestId)
-              await client.send(cast[string](dataMsg))
+            let dataMsg = serializeResult(result, header.requestId)
+            await client.send(cast[string](dataMsg))
             let completeMsg = serializeComplete(result.affectedRows, header.requestId)
             await client.send(cast[string](completeMsg))
           else:
@@ -499,9 +498,8 @@ proc handleClient(server: Server, client: AsyncSocket, clientId: int) {.async.}
           slowQueryLog(slowLog, queryStr, durationMs, clientId)
 
         if success:
-          if result.rows.len > 0:
-            let dataMsg = serializeResult(result, header.requestId)
-            await client.send(cast[string](dataMsg))
+          let dataMsg = serializeResult(result, header.requestId)
+          await client.send(cast[string](dataMsg))
           let completeMsg = serializeComplete(result.affectedRows, header.requestId)
           await client.send(cast[string](completeMsg))
         else: