feat: full BaraQL (GROUP BY/JOIN/CTE/aggregates), Raft consensus, sharding — 107 tests

BaraQL parser extended:
- GROUP BY + HAVING clauses
- JOIN (INNER, LEFT, RIGHT, FULL, CROSS) with ON conditions
- CTE (WITH ... AS) with multiple CTEs
- Aggregate functions (count, sum, avg, min, max)
- CASE/WHEN/THEN/ELSE/END expressions
- BETWEEN, IN, LIKE/ILIKE, IS NULL/IS NOT NULL
- Dotted path identifiers (table.column)
- Subqueries in FROM clause
- CREATE TYPE with colon syntax (name: type)
- UPDATE SET ... WHERE, DELETE FROM ... WHERE

Infrastructure:
- Raft consensus: leader election, log replication, RequestVote/AppendEntries
- Sharding: hash-based, range-based, consistent hashing, rebalancing
- Fixed lexer: = is equality, := is assignment, added : (colon) token
- 34 new parser tests (107 total, all passing)

src/barabadb/query/ast.nim

@@ -120,7 +120,7 @@ type
     case kind*: NodeKind
     of nkSelect:
       selDistinct*: bool
-      selWith*: seq[Node]
+      selWith*: seq[(string, Node)]
       selResult*: seq[Node]
       selFrom*: Node
       selJoins*: seq[Node]