fix(scene): wire KeystrokeProvider around <Setup> so MaskedInput receives keys under rust mode

[esengine]

Closes the keystroke half of the rust-renderer Setup gap. The visible
mirror landed in #998; this one makes the form actually interactive
under `REASONIX_RENDERER=rust`.

Why

Under `REASONIX_RENDERER=rust` the JS process runs Ink with a null
stdin (see `src/cli/ui/scene/null-stdin.ts`). `MaskedInput` was
calling Ink's `useInput`, which subscribes to that null stream, so
keys never reached the form. Combined with the previously-invisible
Ink stdout, a fresh user with no saved key sat at a completely dead
prompt.

What

• `chat.tsx`: the pre-App `` branch is now wrapped in
  `` — the same reader
  the main `` tree below it already uses. So under rust mode
  keys flow from the spawned `reasonix-render --emit-input` child,
  and under the default mode they flow from the singleton
  `StdinReader`.
• `MaskedInput.tsx`: `useInput` → `useKeystroke`. The hook in
  `keystroke-context.tsx` already falls back to Ink's `useInput`
  when no provider is mounted, so any future caller that mounts
  `MaskedInput` outside a provider keeps working unchanged.

That's the whole change — 15 added / 12 removed across two files.

Result

Combined with #998 (`useSetupSceneTrace` visible feedback), the
`REASONIX_RENDERER=rust` fresh-user path now works end-to-end:

• User launches with no `DEEPSEEK_API_KEY` set
• Rust renderer draws the welcome / "enter your DeepSeek API key:"
  / masked-input row in the scene frame
• Keys typed in the terminal flow through the rust input child and
  back into `MaskedInput` via the provider
• `saveApiKey` runs on Enter; Setup resolves; `` mounts

Not in scope

• Stage 3 smoke verification — running the binary on Windows Terminal
  end-to-end to confirm both halves work in practice — is a separate
  step. The unit tests cover the wiring; the UX needs human eyes.
• `pendingReviseEditor` (full-screen editor takeover) is still
  Ink-only; it's a richer modal and gets its own follow-up.

Refs #868