fix: address Copilot review rounds 7 and 8

- rootUnion: a `oneOf` demands that EXACTLY one alternative match, which
  flattening cannot preserve, so it is offered only when a discriminator makes
  the alternatives mutually exclusive; `anyOf` makes no such claim and is
  unaffected. An empty `additionalProperties` schema is the equivalent of
  `true` and no longer declines a union.
- selectBranchIndex: falls back to the branch whose own required fields the
  values supply, so an undiscriminated union's values open the picker on the
  shape they describe rather than on the first branch.
- admitsNull: honors a sibling `nullable: true` beside `const: null`.
- SchemaForm: a value is carried only where the incoming branch leaves the
  root's declaration as it found it; anything it declares itself is reset,
  since it may type the name differently from where the value was typed.
  `hasOwn`/`fromEntries` throughout, so `constructor` is not read as a
  supplied value and `__proto__` is not dropped into the prototype setter.
- TUI: the same prototype-safe construction on the way out of the form, and
  the chosen branch's `required` list is checked at submit — rendering a
  branch's fields optional makes the FORM satisfiable, not the call, so the
  missing names are reported rather than a known-invalid call being sent.

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

README.md

@@ -300,7 +300,7 @@ Open the Tools tab and select `echo`. Above the fields is a **Variant** picker l
 
 Switching branches drops the values that belonged to the outgoing one. They are no longer on screen, so the user can neither see nor clear them, and submitting them would describe a shape the call is not making.
 
-The **TUI** has the same gap and is worth checking against the same server (`--tui`, then test `echo`). ink-form is static — there is no picker to hide the alternatives behind — so each branch becomes its own **section**, preceded by a **Variant** select naming which one the call means. The fields in a branch section are rendered optional whatever the branch says: only one alternative applies to a call, so requiring them would build a form that can never be submitted.
+The **TUI** has the same gap and is worth checking against the same server (`--tui`, then test `echo`). ink-form is static — there is no picker to hide the alternatives behind — so each branch becomes its own **section**, preceded by a **Variant** select naming which one the call means. The fields in a branch section are rendered optional whatever the branch says: only one alternative applies to a call, so requiring them would build a form that can never be submitted. That makes the *form* satisfiable, not the call, so the chosen branch's own `required` list is checked at submit and reported — never sent as a call already known to violate the schema.
 
 The sections are not as independent as they look, which is why the select is not cosmetic: ink-form keeps one value object for the whole form, keyed by field name alone, so two branches both declaring `kind` would be **one** field and the later section's initial value would decide what the earlier one submits. Each branch's fields are therefore rendered under a prefixed name and translated back on submit, where every branch but the chosen one is dropped.
 
@@ -313,7 +313,8 @@ What it declines to flatten is as deliberate as what it flattens, and every case
 - **A union whose members are not all field-carrying object schemas** — including one whose member `type` rules objects out, since tool arguments are a JSON object and such a member can never match. A picker whose options render nothing is no better than no picker.
 - **A branch that restates a constraint the root already states.** The two are conjunctive, so root `minimum: 10` under branch `minimum: 0` is still 10, disjoint `enum`s leave nothing satisfiable, and `type: "string"` under `type: "number"` describes a value that cannot exist — rendering either side would accept what the schema rejects. A property both declare *compatibly* is merged rather than replaced, so a root's `minimum` survives a branch's `maximum`, and a disagreement about `title`/`description` is not a conflict at all.
 - **A composition member stating anything the merge cannot apply.** Only `type`, `properties` and `required` are folded in, so a member carrying a nested `allOf`/`anyOf`, a `not`, an `additionalProperties`, or a `$ref` would have that constraint erased along with the keyword — turning an unsatisfiable schema (`allOf: [false, …]` admits nothing) into a fillable form. `allOf` members are checked against the accumulated merge rather than the root alone, so two of them contradicting each other is caught even when neither contradicts the root.
-- **A union that adds fields under a restrictive root `additionalProperties`.** That keyword constrains whatever its *sibling* `properties` does not name, so a root `additionalProperties: false` rejects every field the branches add — flattening would move them beside the keyword, where they read as allowed.
+- **A `oneOf` whose alternatives are not mutually exclusive.** `oneOf` demands that *exactly one* alternative match, which flattening cannot preserve — the branches are offered as if any would do. It is only safe with a discriminator (a property every branch pins to a `const` of its own), so an undiscriminated `oneOf` is declined. `anyOf` makes no such claim and is offered either way.
+- **A union that adds fields under a restrictive root `additionalProperties`.** That keyword constrains whatever its *sibling* `properties` does not name, so a root `additionalProperties: false` rejects every field the branches add — flattening would move them beside the keyword, where they read as allowed. An empty schema (`{}`) constrains nothing and is treated as permissive.
 - **A schema carrying both `oneOf` and `anyOf`** — independent keywords a value satisfies *together*, not two spellings of one union, so reading one and dropping the other omits real constraints while looking complete. Satisfying both honestly means the cross product of their alternatives, which no real schema has yet asked for.
 - **`not`**, which is not interpreted at all: there is no faithful form for "anything except this".