fix: looseEqual(null, null) returned true, CTE inlining missed distinct/windows

Two SQL correctness bugs:

1. looseEqual() fell through to `return a === b` for nulls, so
   null===null was true. This broke simple CASE: `CASE NULL WHEN NULL
   THEN 'hit' END` incorrectly matched. Added null/undefined guard
   at top of looseEqual — NULL is never equal to anything per SQL.

2. CTE inlining guard didn't check for distinct or window functions.
   A CTE like `WITH t AS (SELECT DISTINCT ...)` would be inlined,
   silently dropping the DISTINCT/window semantics. Added guards.

Author: teamchong

src/sql/compiler.ts

@@ -182,10 +182,12 @@ export function compileFull(stmt: SelectStmt): SqlCompileResult {
     const cte = cteMap.get(desc.table);
     if (cte) {
       const base = cte.descriptor;
-      // Guard: CTEs with sort/limit/aggregates/groupBy/having can't be inlined —
-      // they change the result shape and need materialization first.
+      // Guard: CTEs with sort/limit/aggregates/groupBy/having/distinct/windows
+      // can't be inlined — they change the result shape and need materialization first.
       const canInline = !base.sortColumn && base.limit === undefined &&
         !base.aggregates && !base.groupBy && !cte.havingExpr && !cte.allOrderBy &&
+        !(base.distinct && base.distinct.length > 0) &&
+        !(base.windows && base.windows.length > 0) &&
         !(base.filterGroups && base.filterGroups.length > 0 && desc.filterGroups && desc.filterGroups.length > 0);
       if (canInline) {
         desc.table = base.table;

src/sql/evaluator.ts

@@ -152,6 +152,8 @@ function evaluateUnary(op: string, operandExpr: SqlExpr, row: Row): unknown {
 }
 
 function looseEqual(a: unknown, b: unknown): boolean {
+  // SQL semantics: NULL is never equal to anything (including NULL)
+  if (a === null || a === undefined || b === null || b === undefined) return false;
   if (typeof a === "number" && typeof b === "number") return a === b;
   if (typeof a === "bigint" && typeof b === "bigint") return a === b;
   if (typeof a === "string" && typeof b === "string") return a === b;