fix: address Copilot review round 34 — structured constants, non-required const controls

- A `const` may be an object or an array, with or without a `type`, and
  `String({...})` is "[object Object]" — which no CLI argument can equal, so
  the only value such a schema accepts never matched. Supplied text is parsed
  and compared structurally through the resolver's own comparison (exported as
  `sameJsonValue` so the two cannot disagree), for both branch selection and
  the substitution.
- The TUI's one-option const control is never marked required: its option may
  legitimately be the empty string, which ink-form's required gate can never
  accept, so submission would not even reach `decodeFormValues`. The value is
  fixed by the schema, and `missingRequiredFields` still validates the call.

Signed-off-by: cliffhall <cliff@futurescale.com>

core/json/rootUnion.ts

@@ -240,7 +240,17 @@ function canonicalJson(value: unknown): string {
   return JSON.stringify(value) ?? "undefined";
 }
 
-/** Structural equality, via canonical JSON — enough for schema keyword values. */
+/**
+ * Structural equality, via canonical JSON — enough for schema keyword values.
+ *
+ * Exported because a caller comparing a *supplied* value against a schema's
+ * `const` has to reach the same answer this module does; two implementations
+ * would disagree the moment one of them met an object.
+ */
+export function sameJsonValue(a: unknown, b: unknown): boolean {
+  return sameValue(a, b);
+}
+
 function sameValue(a: unknown, b: unknown): boolean {
   return a === b || canonicalJson(a) === canonicalJson(b);
 }