Add keyboardType, submitLabel, and onSubmit

[colemancda]

The form-input batch for text entry: .keyboardType(_:), .submitLabel(_:), and .onSubmit(perform:). All three are read by the field off its own modifier list, the same way .focused already is.

Mapping

• keyboardType → Compose KeyboardType (.decimalPad→Decimal, .numberPad→Number, .emailAddress→Email, .URL→Uri, .phonePad→Phone, else Text). UIKeyboardType is defined with SwiftUI's spelling so call sites port unchanged.
• submitLabel → the field's ImeAction (.search→Search, .go→Go, .send→Send, .next→Next, else Done), which names the keyboard's action key.
• onSubmit → a void callback wired to every KeyboardActions handler, so it fires whatever action key the submit label selected.

The field is now singleLine = true, which is what makes the IME show an action key rather than a newline key.

Verification

• swift test — 2 new tests (type/label spelling emit; onSubmit registers a callable callback), 112 passing.
• Emulator, verified against the real IME, not just the render:
  • focusing the .decimalPad field, dumpsys input_method reported inputType=0x2002 — TYPE_CLASS_NUMBER | TYPE_NUMBER_FLAG_DECIMAL, i.e. the decimal pad reached the IME
  • typing in the .submitLabel(.search) field and pressing the action key fired onSubmit, incrementing the on-screen counter to 1

Scope

The common keyboard types and submit labels; exotic UIKeyboardType cases fall back to the text keyboard. onSubmit(of:) (the search-vs-text trigger variant) isn't modeled — this is the plain onSubmit. Note the submitted-text binding can lag by a keystroke if the last onChange hasn't round-tripped before submit reads it — the async text-echo path, not specific to this change.