TUI: upgrade OpenTUI 0.1.87->0.3.0 + Plugin Slots/screen modes (gated, #212 P2, phase 6)

[windoliver]

Phase 6 of the #212 OpenTUI adoption plan. Design spec: docs/superpowers/specs/2026-05-30-tui-opentui-adoption-design.md.

Gated tracking issue — no implementation until the bump is validated. These are the only #212 features genuinely missing from the pinned 0.1.87; latest published is 0.3.0 (a 0.1 → 0.3 minor jump) where they most likely live.

Scope

1. Audit 0.1.87 → 0.3.0 breaking changes against the diff/code/markdown/scrollbox APIs phases 1–5 depend on; bump and re-verify the export surface.
2. Then, if present in 0.3.0:
   • Plugin Slots ([Epic] TUI: leverage OpenTUI built-in components (diff, code, scrollbox, markdown) #212 P2-7) — createSlotRegistry/slot modes, runtime panel registration.
   • Formal screen modes split-footer / capture-stdout ([Epic] TUI: leverage OpenTUI built-in components (diff, code, scrollbox, markdown) #212 P2-9) — currently only experimental_splitHeight.
   • Renderer focus/blur events ([Epic] TUI: leverage OpenTUI built-in components (diff, code, scrollbox, markdown) #212 P2-11 remainder) — only theme_mode+selection exist in 0.1.87.

Acceptance

• Upgrade lands with phases 1–5 suites still green against 0.3.0.
• Gated features either implemented (if 0.3.0 ships them) or documented as still-missing upstream.

Part of #212.