fix: union struct-literal init (issue 0158)

A plain union initialized with a struct literal (b : Overlay = .{ f = 3.14 })
silently miscompiled — it fell through the generic struct-literal path
(getStructFields returns empty for a union), building a malformed structInit
whose overlapping zero-fill clobbered the named member, so it read back 0.0
(and a type-pun read segfaulted).

lowerStructLiteral now detects a plain-union target and dispatches to a new
lowerUnionLiteral, which writes each named member into a union-sized slot via
the same lvalue resolver the u.member = v assignment path uses, then loads the
union value back. Validity: the named members must share one arm — a single
direct member, or several promoted members of the same anonymous-struct variant.
Overlapping members, members from different arms, and positional union literals
are rejected with a diagnostic (no silent last-wins); an empty .{} yields an
undefined union (matching the --- form).

specs.md updated. Regressions: examples/types/0194 (valid forms) +
examples/diagnostics/1191 (overlap rejection).

src/ir/lower/expr.zig

@@ -86,6 +86,15 @@ pub fn lowerStructLiteral(self: *Lowering, sl: *const ast.StructLiteral, span: a
         self.resolveTypeWithBindings(te)
     else self.target_type orelse .unresolved;
 
+    // Plain (untagged) union target: build by writing each named member into a
+    // union-sized slot. `getStructFields` returns empty for a union, so the
+    // generic struct path below would emit a malformed `structInit` whose
+    // overlapping zero-fill clobbers the named member (issue 0158). Tagged
+    // unions were already handled above.
+    if (!ty.isBuiltin() and self.module.types.get(ty) == .@"union") {
+        return self.lowerUnionLiteral(sl, ty, span);
+    }
+
     // Get struct field types for coercion and ordering
     const struct_fields = self.getStructFields(ty);