fix: address Copilot review round 41 — const does not demand a property, required-only branches

- `const` constrains a property that is PRESENT; it neither requires the
  property nor acts as a default. It is now supplied automatically only where
  the schema also requires the field — in `collectSchemaDefaults`,
  `applySchemaConstants` and the TUI's initial values alike — so an optional
  `dryRun: { const: true }` no longer turns a valid `{}` call into one that
  asks the server to do something. A value that IS present is still corrected
  to the constant, whatever supplied it, and the field stays read-only.
- Offerability is judged on the base+member merge rather than the raw member,
  so `anyOf: [{ required: ["email"] }, { required: ["phone"] }]` over
  root-declared properties is offered. It was declined, which left the gate
  checking the base alone and accepting `{}` — which that schema rejects.
- The TUI's branch sections keep the schema's own `required` for the seeding
  decision while still rendering every branch field non-required, which is what
  keeps a static form submittable.

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

clients/web/src/test/core/rootUnion.test.ts

@@ -617,6 +617,24 @@ describe("resolveRootUnion", () => {
     });
   });
 
+  it("offers a branch that only adds required names over the root's fields", () => {
+    // `anyOf: [{ required: ["email"] }, { required: ["phone"] }]` is an
+    // ordinary way to say "one of these two". Judging the member on its own
+    // properties declined it, leaving the gate checking the base alone and
+    // accepting `{}` — which the schema rejects.
+    const { branches } = resolveRootUnion({
+      type: "object",
+      properties: { email: { type: "string" }, phone: { type: "string" } },
+      anyOf: [
+        { type: "object", required: ["email"] },
+        { type: "object", required: ["phone"] },
+      ],
+    });
+    expect(branches).toHaveLength(2);
+    expect(branches[0].schema.required).toEqual(["email"]);
+    expect(branches[1].schema.required).toEqual(["phone"]);
+  });
+
   describe("oneOf exclusivity", () => {
     it("offers a discriminated oneOf", () => {
       expect(