ffi any_to_string handles optionals — make-green

Closes the optional-through-Any gap that test 178 pinned.

Stdlib (`library/modules/std.sx`):
- New `optional_to_string :: (o: $T) -> string` returns `"null"`
  when the optional is None, otherwise recurses through
  `any_to_string` on the unwrapped inner value. Per-shape
  monomorphisation re-emits this for each concrete `?T`.
- `any_to_string` grows a `case optional:` arm that dispatches
  through `cast(type) val` (same shape as `case struct:` etc.).
  The cast picks up the dynamic optional type from the Any tag.

Compiler (`src/ir/lower.zig`):
- `resolveTypeCategoryTags` recognises "optional" as a dynamic
  category, scanning the TypeTable for `info == .optional`. The
  type-switch dispatch then routes any ?T tag into the optional
  arm.

IR snapshots regenerated where the optional addition shifted
constant pool / string numbering: 142, ffi-objc-call-06,
ffi-objc-dsl-07. 218/218 (test 178 included).

The variadic auto-unwrap in `packVariadicCallArgs` stays in
place — direct `print(opt)` calls still flow through it. The new
arm closes the gap for struct fields, slice elements, and any
other path that boxes an optional before stringifying.

src/ir/lower.zig

@@ -9491,7 +9491,7 @@ pub const Lowering = struct {
         }
 
         // Dynamic categories: scan TypeTable for matching types
-        const Category = enum { @"struct", @"enum", @"union", slice, array, pointer, vector };
+        const Category = enum { @"struct", @"enum", @"union", slice, array, pointer, vector, optional };
         const cat: ?Category = if (std.mem.eql(u8, name, "struct"))
             .@"struct"
         else if (std.mem.eql(u8, name, "enum") or std.mem.eql(u8, name, "union"))
@@ -9504,6 +9504,8 @@ pub const Lowering = struct {
             .pointer
         else if (std.mem.eql(u8, name, "vector"))
             .vector
+        else if (std.mem.eql(u8, name, "optional"))
+            .optional
         else
             null;
 
@@ -9517,6 +9519,7 @@ pub const Lowering = struct {
                     .array => info == .array,
                     .pointer => info == .pointer or info == .many_pointer,
                     .vector => info == .vector,
+                    .optional => info == .optional,
                 };
                 if (matches) {
                     tags.append(self.alloc, @intCast(idx)) catch {};