feat(scene): mirror the API-key Setup form into the scene frame

[esengine]

First step toward un-blocking the fresh-user path under
`REASONIX_RENDERER=rust`. Direction-B follow-on to #997.

Why

The Ink-side `Setup` screen rendered to the null stdout, so a user
with no saved API key saw nothing at all and had no way to know the
loop was waiting on their key. The scene producer now emits a
parallel setup frame so the prompt is visible:

```
✦ reasonix · welcome
Enter your DeepSeek API key:
get one at https://platform.deepseek.com
❯ •••••▮
✗ (only when error is set)
Ctrl+C to exit · /exit to quit
```

What

• New `buildSetupFrame(input, cols, rows)` — completely separate
  from `buildTraceFrame`. The setup screen never coexists with
  cards / status / composer, so a shared layout would only constrain
  both.
• New `useSetupSceneTrace` hook — `Setup.tsx` calls it with
  `bufferLength` and the current `error` string.
• Only the buffer length crosses the scene boundary. Raw key
  material never gets serialized into the scene stream — the scene
  layer never sees the API key, just the count of dots to render.

Known limitation (next PR)

No keystroke change in this PR — `MaskedInput` keeps using Ink's
`useInput`. Under `REASONIX_RENDERER=rust` Ink is wired to the
null stdin (see `src/cli/ui/scene/null-stdin.ts`), so `useInput`
receives nothing and the user still can't actually type a key
without falling back to `REASONIX_RENDERER`-unset mode. Closing
that gap means wiring `KeystrokeProvider` around the pre-App
`` mount and routing the rust input child's keys to
`MaskedInput`. That's a separate, scoped follow-up.

This PR is intentionally just the visible-feedback half — the user
at least learns "the loop wants my API key" instead of staring at
a blank terminal.

Tests

`tests/scene-trace-frame.test.ts` — 5 new cases:

• welcome / prompt / placeholder / exit hint when buffer is empty
• `•` dots + `▮` cursor when the user has typed
• buffer-length-only crossing the boundary (5 chars → exactly 5 `•`)
• error row inserted above the exit hint when an error is set
• error row omitted when error is undefined

`npm run verify` green via prepush gate.

Refs #868