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.

src/ir/lower.zig

@@ -1607,6 +1607,7 @@ pub const Lowering = struct {
     pub const lowerConstDecl = lower_stmt.lowerConstDecl;
     pub const lowerReturn = lower_stmt.lowerReturn;
     pub const lowerAssignment = lower_stmt.lowerAssignment;
+    pub const fieldLvalueResolve = lower_stmt.fieldLvalueResolve;
     pub const fieldLvaluePtr = lower_stmt.fieldLvaluePtr;
     pub const lowerExprAsPtr = lower_stmt.lowerExprAsPtr;
     pub const storeOrCompound = lower_stmt.storeOrCompound;