/layout deterministic showcase

[raiseCatError]

Closes #133

Behavior

• /layout (also /settings → Config → Layout) opens a panel on the shared panel shell with two rows: Composer position ‹Bottom | Top | Flow› and Transcript ‹Normal | Chat›. ↑↓ move, ←→ change, Enter saves and applies live, and Esc cancels. When the draft differs, the rows show saved: hints and the header says unsaved preview.
• Preview: a deterministic mini-screen of a fixed sample (a folded build, a multi-line command, a failure, and a running command with its activity row).
  • It renders through the real transcript presenter and planScreen. Only the prompt and input text are placeholders, and completion times use a fixed date.
  • The sample lives in its own OutputBuffer, so nothing runs and nothing reaches the transcript, journal or /copy.
• Sizing: the preview is sized to the whole sample plus spare rows when the terminal allows, so Flow's prompt visibly follows the output while Bottom stays docked. A caption describes the selected position. Very short terminals get Enlarge the window to see the preview.

Composition with Bottom / Top / Flow and Chat

• It edits the same composerPosition and transcriptPresentation settings as the Config rows and the palette toggles. New shared value/label lists in configuration.ts feed both the Config rows and the panel, so there's one configuration and only implemented values ever appear.
• Saving goes through the existing applySettingsConfiguration: it persists the settings, sets the presenter layout live, and the next planScreen uses the new position.
• Flow, Top and Bottom geometry is untouched. The preview calls the same planScreen, including Flow's stack.

Notes

• The Layout entry is appended at the end of the Settings list, so existing entries keep their positions.
• /layout deterministic showcase #133's "hide unimplemented combinations" is now moot, since all 3 × 2 exist after Composer position: Flow / Classic #132. The shared value lists keep it true going forward.
• Also hardened the Composer position: Flow / Classic #132 Flow end-to-end test: it resends its exit key until the fullscreen app takes it. It failed once under load.

Tests

• tests/layoutPanel.test.ts:
  • A snapshot of all six combinations in tests/fixtures/layout-preview.txt (regenerate with UPDATE_SNAPSHOTS=1), with determinism checked including styling.
  • What distinguishes each combination: Top composer first, Bottom docked, Flow after the running command, Chat right-aligning history while the live composer stays normal. Every run also checks the fold, the multi-line command, the failure and the activity row are present.
  • Keys and unsaved hints; sizing at 80×60 down to 24×10, never exceeding the width.
  • The slash command and the settings destination.
  • App level: previewing leaves the transcript and /copy untouched, and Enter persists (to a temporary config dir), applies the presentation live and switches the plan to Flow. Esc discards.
• Existing Dock Top, Flow, settings, palette and configuration tests pass.
• Build, typecheck and git diff --check are clean. The last local full runs passed 586/586, except one miss by the pre-existing suggestion-latency benchmark under load.

Limitations

• On terminals shorter than about 45 rows, the preview can't show the whole sample. Flow then looks like Bottom, which is also what a real full screen does, and the caption explains the difference.
• The preview's prompt and input are placeholders, not the user's configured prompt provider (/prompt has its own showcase).
• Physical validation is deferred to the v0.6 test pass.