v3.1.0 — Step-view lifetime moves into the widget tree

Moves step-view lifetime into the widget tree, which is the root cause behind
the disposal bug 3.0 patched over. No breaking changes: the InputRegistry
contract, FormStepView and BaseStepView all keep their shape, and the 30
built-in inputs needed no edits.

Changed

• FormStepView is a StatefulWidget. It was a StatelessWidget holding
  controllers, which has no disposal lifecycle at all — so dispose() was only
  ever called by FormStackForm's own bookkeeping, and only for views it
  happened to be holding. Its State now builds the view and disposes it when
  it leaves the tree. Because the state stays on the widget, every existing
  subclass compiles unchanged.
• Step subtrees are keyed by step. Consecutive steps commonly use the same
  view class; without a differing key Flutter reconciles them onto one element,
  reuses the State, and never disposes the outgoing view — leaking its
  controllers and carrying the previous step's focus into the next step.
• The view cache is gone. FormStackForm keeps a reference to the view for
  the step on screen and nothing more. A retained view would be reused after
  the framework had disposed it, so the cache and framework ownership cannot
  coexist. Answers are unaffected: they live on formStep.result, which is
  written before every navigation.

Fixed

• A captured signature was lost on back navigation. InputType.signature
  never restored from formStep.result, so the answer survived only as long as
  the view cache happened to hold the view. It now restores and shows the
  captured image. This was already reachable in 3.0 by setting
  maxCachedViews: 0, which the docs suggested.

Deprecated

• FormStackForm.maxCachedViews, clearViewCache() and disposeViews() are
  no-ops and will be removed in 4.0. Nothing is retained to bound, clear or
  dispose. Existing calls are harmless.

Added

• A round-trip test over every buildable input type: set an answer, navigate
  away, navigate back, advance, and assert the answer survived. An input that
  fails to restore reports an empty resultValue() on the way forward and
  destroys what the user entered — exactly what the cache was hiding. Verified
  to fail when signature restoration is removed.