fix: address Copilot review round 13 — cross-keyword contradictions, own-property required check

- rootUnion: keywords the two declarations state SEPARATELY can contradict too.
  A merged `const` is now checked against its own `type` and `enum`, so
  root `x: { type: "string" }` under branch `x: { const: 1 }` declines
  instead of seeding an immutable 1 into a string field. An integer const still
  satisfies a `number` type — the one direction JSON Schema widens.
- TUI missingRequiredFields tests `Object.hasOwn` first: an argument legally
  named `constructor` resolved to the inherited one and read as supplied, and
  the call went out without it.

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

clients/tui/src/utils/schemaToForm.ts

@@ -462,6 +462,10 @@ export function missingRequiredFields(
       ? base
       : branches[selectedBranchIndex(base, branches, rawValues)]!.schema;
   return (effective.required ?? []).filter((name) => {
+    // `hasOwn` first: an argument legally named `constructor` would otherwise
+    // resolve to the inherited one and read as supplied, and the call would go
+    // out without it.
+    if (!Object.hasOwn(decoded, name)) return true;
     const value = decoded[name];
     return value === undefined || value === "";
   });