fix: address Copilot review round 4 — allOf faithfulness, const null, branch-aware seeding

- rootUnion: an `allOf` member is foldable only when it states nothing beyond
  what the merge applies (`type`/`properties`/`required` plus annotations),
  so a nested `anyOf`, a `not`, or an `additionalProperties` can no longer
  be erased along with the keyword; members are checked against the
  ACCUMULATED merge, so two of them contradicting each other is caught even
  when neither contradicts the root.
- admitsNull: `const: null` admits null, so a required field pinned to it is
  no longer seeded with the one value it accepts and then reported missing.
- collectSchemaDefaults takes the values a caller is about to overlay, and
  seeds the branch THEY identify — the App deep link passes its `appArgs`, so
  branch 0's defaults can no longer sit invisibly under branch 1's arguments.
- The branch inference moves into core as `selectBranchIndex`, shared by the
  seeding and the form's picker so the two cannot disagree.

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

core/json/nullableUnion.ts

@@ -372,6 +372,13 @@ export function admitsNull(schema: NullableUnionSchema): boolean {
   if (nullExcludedBySiblings(schema)) {
     return false;
   }
+  // `const: null` admits null and nothing else, whatever the schema says (or
+  // omits) about `type`. Without this a required field pinned to null is seeded
+  // with the only value it accepts and then reported as missing, leaving submit
+  // permanently disabled on a form the user cannot change (#2123).
+  if (schema.const === null) {
+    return true;
+  }
   // Applicators this module does not evaluate. `not: { type: "null" }` rules
   // null out; `allOf` can add a member that does; and `oneOf` requires
   // **exactly one** branch to match, so a null branch does not by itself mean