feat(scene): render approval/confirm modals as a y/n stub row

[esengine]

Fourth sub-PR under the rust direction-B plan (#868).

What

Under `REASONIX_RENDERER=rust` the various pending* confirmation
modals — shell command, sandbox path, edit review, free-form choice,
plan approval — had no scene representation. The user saw the
composer prompt while the loop was actually waiting on a y/n. The
composer is now replaced with a stub row whenever any of those
states is set:

```
❓ [shell] rm -rf /tmp/x [y/n]
```

• New `SceneTraceInput.approvalKind` / `approvalPrompt` scalar pair
  — primitives keep effect deps stable (no JSON encoding needed for
  one object with two strings).
• `buildTraceFrame`: when `approvalPrompt` is set, push `approvalRow`
  in place of `composerRow`, and suppress the slash overlay so the
  modal owns the bottom of the frame.
• Prompt clipped to 60 chars with a trailing ellipsis so a long
  shell command stays readable.
• `App.tsx` derives the pair via `useMemo` from `pendingShell` /
  `pendingPath` / `pendingEditReview` / `pendingChoice` /
  `pendingPlan`, in that priority order.

Deliberately a stub

Per-modal refinement (the choice options for `pendingChoice`, a diff
preview for `pendingEditReview`, the full plan text and Approve/Refine/
Cancel for `pendingPlan`, etc.) lives in later sub-PRs — this is the
"smallest visible win" so the user at least knows the loop is blocked
on a yes/no question.

Also skipped on purpose: `pendingSessionsPicker` /
`pendingCheckpointPicker` / `pendingMcpHub` (list-pickers, get their
own scene step — handoff step #5), `pendingReviseEditor` (full editor
takeover, owns the screen), `stagedInput` / `stagedChoiceCustom`
(free-form input where the composer should remain visible).

Tests

`tests/scene-trace-frame.test.ts` — 4 new cases:

• approval row replaces composer; the run sequence contains the kind
  tag, the prompt, and the `[y/n]` suffix
• overlong prompts are clipped to 60 chars with an ellipsis
• kind tag is omitted when `approvalKind` is undefined
• slash overlay is suppressed while an approval is active

`npm run verify` green via prepush gate.

Refs #868