fix: address Copilot review round 15 — unevaluable const siblings, $ref fields, per-branch draft keys

- rootUnion: a merged declaration pairing a `const` with an assertion this
  module cannot evaluate (`minimum`, `pattern`, …) is declined. Proving the
  conjunction safe is the requirement, not disproving it — `minimum: 10`
  beside `const: 1` is as unsatisfiable as a type mismatch.
- declaresAnyFields counts a `$ref`: its shape is unknown rather than empty,
  so `anyOf: [{ $ref }, { $ref }]` no longer reports an App tool as
  input-free and auto-invokes it with `{}`.
- SchemaForm keys its draft-holding fields by the active branch as well, so a
  half-typed `-` or an unparsed JSON draft cannot survive a switch into a
  same-named field of the incoming branch — with both parent values
  `undefined`, nothing else tells the field the entity changed.

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

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

@@ -402,6 +402,20 @@ describe("resolveRootUnion", () => {
     expect(branches).toEqual([]);
   });
 
+  it("declines a const paired with an assertion it cannot evaluate", () => {
+    // `minimum: 10` beside `const: 1` is as unsatisfiable as a type mismatch,
+    // and proving the conjunction safe is the requirement here.
+    const { branches } = resolveRootUnion({
+      type: "object",
+      properties: { x: { type: "number", minimum: 10 } },
+      anyOf: [
+        { type: "object", properties: { x: { const: 1 } } },
+        { type: "object", properties: { other: { type: "string" } } },
+      ] as unknown[],
+    });
+    expect(branches).toEqual([]);
+  });
+
   it("accepts a const its root type and enum admit", () => {
     const { branches } = resolveRootUnion({
       type: "object",