lang: fix struct-field null/undef over-store (issue 0154)

Assigning null/--- to a struct field picked up a leaked enclosing
target_type (the function's return type, set for the whole body), so
constNull/constUndef built a whole-struct-typed value. The oversized
store overran the field's slot and clobbered the saved frame pointer,
so the function returned to 0x0. Surfaced building a by-value-returned
struct whose array field precedes a pointer field (Scheduler.init()).

Fix: add null_literal/undef_literal to the needs_target switch in
lowerAssignment so the field's own type is used. Regression:
examples/types/0193-types-sret-array-before-pointer.sx.

src/ir/lower/stmt.zig

@@ -629,7 +629,14 @@ pub fn lowerAssignment(self: *Lowering, asgn: *const ast.Assignment) void {
         // unchanged into method-call arg slots (`resolveCallParamTypes` can't
         // override target_type per-arg).
         const needs_target = switch (asgn.value.data) {
-            .enum_literal, .struct_literal, .tuple_literal, .if_expr, .match_expr, .block, .unary_op, .binary_op => true,
+            // `null` / `---` (undef) carry NO type of their own — they take the
+            // store slot's type from `target_type`. Without setting it to the
+            // field type here, a leaked enclosing `target_type` (e.g. the
+            // function's return type while lowering its body, decl.zig:2691)
+            // reaches `constNull`/`constUndef` and builds a WHOLE-STRUCT-typed
+            // null, emitting an oversized store that overruns the field's slot
+            // and corrupts neighboring stack (issue 0154).
+            .enum_literal, .struct_literal, .tuple_literal, .if_expr, .match_expr, .block, .unary_op, .binary_op, .null_literal, .undef_literal => true,
             .call => |vc| vc.callee.data == .enum_literal,
             else => false,
         };