fix: address Copilot review round 37 — stale constants, inherited answers, prefixed labels

- The seeding effect re-applies constants as well as filling absences: an
  in-place schema change can move a `const` the user cannot edit, and the
  read-only field then displayed the new value while `values` still held the
  old one — which is what would be submitted. Ordinary defaults are still only
  ever added, so an edited field keeps what the user put there.
- hasMissingIn checks own-property presence: a required argument legally named
  `constructor` resolved to the inherited one and read as supplied, enabling
  a submit the schema rejects.
- The TUI keeps the declared name as a branch field's fallback title. The
  prefix is an internal field NAME, and buildFields labels from its map key —
  so the showcase displayed `__b0__address` where the schema says
  `address`.

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

clients/web/src/utils/jsonUtils.test.ts

@@ -601,6 +601,20 @@ describe("root composition (#2123)", () => {
     ).toBe(true);
   });
 
+  it("does not read an inherited property as a supplied answer", () => {
+    // `constructor` is a legal argument name; reading it off the prototype
+    // would enable a submit the schema rejects.
+    const schema: InspectorFormSchema = {
+      type: "object",
+      // Built through `fromEntries`: in an object literal, `constructor` is
+      // TypeScript's own inherited member rather than a plain key.
+      properties: Object.fromEntries([["constructor", { type: "string" }]]),
+      required: ["constructor"],
+    };
+    expect(hasMissingRequiredFields(schema, {})).toBe(true);
+    expect(hasMissingRequiredFields(schema, { constructor: "x" })).toBe(false);
+  });
+
   it("blocks submission while no branch is satisfied", () => {
     expect(hasMissingRequiredFields(UNION, {})).toBe(true);
     expect(hasMissingRequiredFields(UNION, { kind: "email" })).toBe(true);