Phase 7.3 strangler reorg — add op-app, the thin crate that names the
editor application's composition root.
Investigation found no shared host bootstrap left to extract: the
editor-UI composition (widgets, theme, layout scene) already lives in
op-editor-ui, and each host (op-host-native / op-host-web) owns only
platform-specific backend wiring. So per YAGNI op-app stays thin — it
re-exports op-editor-ui plus the per-platform host entry point behind
its target cfg, and documents the composition. If real cross-host
wiring later emerges, it lands here.
Builds green on both native and wasm32-unknown-unknown.
