fix: propagate union-member type to a struct-literal RHS

Assigning a struct literal to a named-struct member of a plain union
(`u.b = .{ ... }`) lowered the RHS as .unresolved and tripped the
LLVM-emission tripwire: lowerAssignment's .field_access target-type
path used getStructFields, which returns nothing for a union, so the
literal never received its target type.

Unify the lvalue field matcher into a pure fieldLvalueResolve consumed
by both fieldLvaluePtr (GEP builder) and the target-type path, so the
store slot and the RHS target type can't diverge (covers union direct +
promoted members, tuple/vector lanes, and structs).

Resolves issue 0133 (depended on 0135). Regression test: examples/0184.
Notes the now end-to-end union path in issue 0132.

examples/0184-types-union-member-struct-literal-assign.sx

@@ -0,0 +1,22 @@
+// Assigning a struct LITERAL to a named-struct member of a plain `union`.
+// `u.b = .{ code = 9 }` types the literal as the union member's struct type
+// `S` and stores it — the target type propagates to a union-member lvalue
+// exactly as it does to a struct field.
+//
+// Regression (issue 0133): the literal used to lower as `.unresolved` (the
+// target-type path only inspected struct fields, not union members) and trip
+// the LLVM-emission tripwire in emitStructInit.
+
+#import "modules/std.sx";
+
+S :: struct { code: i64; }
+U :: union { a: i64; b: S; }
+
+main :: () {
+    u : U = ---;
+    u.b = .{ code = 9 };          // union member <- struct literal
+    print("code={}\n", u.b.code); // 9
+
+    u.a = 5;                      // scalar member still works
+    print("a={}\n", u.a);         // 5
+}

examples/expected/0184-types-union-member-struct-literal-assign.exit

@@ -0,0 +1 @@
+0

examples/expected/0184-types-union-member-struct-literal-assign.stderr

@@ -0,0 +1 @@
+

examples/expected/0184-types-union-member-struct-literal-assign.stdout

@@ -0,0 +1,2 @@
+code=9
+a=5