ffi any_to_string optional dispatch — expected-failing lock-in

`examples/178-any-to-string-optional.sx` prints a struct whose
three fields are `?s64` / `?string` / `?bool`, in both Some and
None form. The struct-print path goes through `field_value(s, i)
-> Any` and then `any_to_string(Any)`. Today: `any_to_string`
has no `case optional:` arm and `resolveTypeCategoryTags` has no
"optional" category — every optional field falls through to the
`<?>` default. Expected output captures the working post-fix
form (`a: 42`, `b: hi`, `c: true` for Some; `null` across the
board for None).

The next commit adds `optional_to_string` + `case optional:` to
std and "optional" to `resolveTypeCategoryTags`. Variadic
auto-unwrap (`packVariadicCallArgs`) keeps printing direct
`print(opt)` calls correctly today; this fix closes the gap for
struct fields, slice elements, and anywhere else an optional
flows through Any.

examples/178-any-to-string-optional.sx

@@ -0,0 +1,31 @@
+// any_to_string didn't handle optionals. A struct field of type `?T`
+// printed as `<?>` (any_to_string's "no case matched" default)
+// because there was no `case optional:` arm, and no dispatch table
+// entry mapping `?T` TypeIds to the optional category.
+//
+// The variadic auto-unwrap path (packVariadicCallArgs) papered over
+// this for direct `print("{}\n", opt)` calls — it stringified
+// optionals to either the inner value's repr or `"null"` BEFORE
+// boxing as Any. But anywhere else that boxes an optional and reads
+// it back through any_to_string (struct field printing,
+// `xx opt : Any`, future user code) hit the `<?>` floor.
+//
+// Locks in the fix: each ?T variant routes through `case optional:`
+// → `optional_to_string(cast(type) val)` → either the inner value's
+// `any_to_string` representation or the literal `"null"`.
+
+#import "modules/std.sx";
+
+S :: struct {
+    a: ?s64;
+    b: ?string;
+    c: ?bool;
+}
+
+main :: () {
+    s1 := S.{ a = 42, b = "hi", c = true };
+    print("{}\n", s1);
+    s2 := S.{ a = null, b = null, c = null };
+    print("{}\n", s2);
+    0;
+}

tests/expected/178-any-to-string-optional.exit

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

tests/expected/178-any-to-string-optional.txt

@@ -0,0 +1,2 @@
+S{a: 42, b: hi, c: true}
+S{a: null, b: null, c: null}