fix: address Copilot review round 35 — reseed on an in-place schema change, label every primitive const

- The seeding effect is keyed by the fixed values themselves as well as the
  entity and branch. A tool refreshed in place keeps its `resetKey` and its
  branch while its schema changes underneath, so a newly pinned field was
  rendered read-only and never seeded — leaving a required one permanently
  unsubmittable. The schema object's identity says nothing (callers rebuild it
  every render), which is why the key is derived from what has to be seeded.
- branchLabel stringifies every primitive constant. `hasDiscriminator`
  accepts distinct booleans and nulls, so a `true`/`false` union was
  discriminated perfectly well and still labelled "Option 1"/"Option 2".

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

core/json/rootUnion.ts

@@ -586,20 +586,27 @@ function branchLabel(
     return branch.title;
   }
   const properties = propertiesOf(branch);
-  const constOf = (name: string): string | null => {
+  // Every primitive constant is a usable label, not just the string and number
+  // cases: `hasDiscriminator` accepts distinct booleans and nulls, so a
+  // `true`/`false` union would otherwise be labelled "Option 1"/"Option 2"
+  // while being discriminated perfectly well. An object or array constant is
+  // not a label — `undefined` marks "nothing usable here", which lets a `null`
+  // constant be a value rather than the sentinel.
+  const constOf = (name: string): string | undefined => {
     const property = toBranch(properties[name]) as { const?: unknown } | null;
-    const value = property?.const;
-    return typeof value === "string" || typeof value === "number"
+    if (property === null || !("const" in property)) return undefined;
+    const value = property.const;
+    return value === null || typeof value !== "object"
       ? String(value)
-      : null;
+      : undefined;
   };
   if (discriminatorProperty !== undefined) {
     const value = constOf(discriminatorProperty);
-    if (value !== null) return value;
+    if (value !== undefined) return value;
   }
   const constants = Object.keys(properties)
     .map((name) => constOf(name))
-    .filter((value): value is string => value !== null);
+    .filter((value): value is string => value !== undefined);
   if (constants.length === 1) return constants[0];
   return `Option ${index + 1}`;
 }