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/lexer.nim

@@ -107,6 +107,7 @@ type
     tkAssign
     tkArrow
     tkDoubleColon
+    tkColon
     tkDot
     tkComma
     tkSemicolon
@@ -351,7 +352,7 @@ proc nextToken*(l: var Lexer): Token =
       discard l.advance()
       return Token(kind: tkEq, value: "==", line: startLine, col: startCol)
     discard l.advance()
-    return Token(kind: tkAssign, value: ":=", line: startLine, col: startCol)
+    return Token(kind: tkEq, value: "=", line: startLine, col: startCol)
   of ':':
     if l.pos + 1 < l.input.len and l.input[l.pos + 1] == '=':
       discard l.advance()
@@ -362,7 +363,7 @@ proc nextToken*(l: var Lexer): Token =
       discard l.advance()
       return Token(kind: tkDoubleColon, value: "::", line: startLine, col: startCol)
     discard l.advance()
-    return Token(kind: tkInvalid, value: ":", line: startLine, col: startCol)
+    return Token(kind: tkColon, value: ":", line: startLine, col: startCol)
   of '!':
     if l.pos + 1 < l.input.len and l.input[l.pos + 1] == '=':
       discard l.advance()