feat(selfhost): add &T (shared ref) and &mut T (mutable ref) type syntax

- Add tekRef and tekMutRef to TypeExpr enum (ast.bux)
- Parse &T and &mut T in parserParseType, before *T handling (parser.bux)
- Map tekRef/tekMutRef to tyPointer in HIR lowering (hir_lower.bux)
- Both emit as T* in C backend (same as raw pointer)
- Borrow checker rejects write-through-pointer in @[Checked] functions
  for all pointer types (safe default)

src/hir_lower.bux

@@ -53,7 +53,7 @@ func Lcx_ResolveTypeKindFromName(name: String) -> int {
 func Lcx_ResolveTypeKind(te: *TypeExpr) -> int {
     if te == null as *TypeExpr { return tyUnknown; }
 
-    if te.kind == tekPointer { return tyPointer; }
+    if te.kind == tekPointer || te.kind == tekRef || te.kind == tekMutRef { return tyPointer; }
     if te.kind == tekSlice { return tySlice; }
     if te.kind == tekTuple { return tyTuple; }
     if te.kind == tekFunc { return tyFunc; }