feat: JSON/JSONB validation, multi-column indexes, CTE execution, wire protocol column types

- Add fkJson to wire protocol with serialization/deserialization
- Validate JSON/JSONB on INSERT/UPDATE via std/json
- Send real column type metadata in wire protocol responses
- Update all 4 clients (Nim, Python, JS, Rust) for JSON and columnTypes
- Implement multi-column index parser (CREATE INDEX idx ON t(a, b))
- Add ciColumns to AST nkCreateIndex
- Build composite index keys as table.col1.col2 with val1|val2
- Support multi-column exact match in SELECT for AND chains
- Implement non-recursive CTE execution via ctx.cteTables materialization
- Add tkRecursive token and parse WITH RECURSIVE
- Fix test isolation: use temp dirs for JOIN, migration, and RLS tests
- All tests passing (0 failures)

src/barabadb/query/ast.nim

@@ -149,7 +149,7 @@ type
     case kind*: NodeKind
     of nkSelect:
       selDistinct*: bool
-      selWith*: seq[(string, Node)]
+      selWith*: seq[(string, Node, bool)]
       selResult*: seq[Node]
       selFrom*: Node
       selJoins*: seq[Node]
@@ -286,6 +286,7 @@ type
     of nkCreateIndex:
       ciTarget*: string
       ciName*: string
+      ciColumns*: seq[string]
       ciExpr*: Node
       ciKind*: IndexKind
     of nkDropIndex:
@@ -309,7 +310,7 @@ type
     of nkReturning:
       retExprs*: seq[Node]
     of nkWith:
-      withBindings*: seq[(string, Node)]
+      withBindings*: seq[(string, Node, bool)]
     of nkBinOp:
       binOp*: BinOpKind
       binLeft*: Node