Engine-side fixes surfaced by the TinySkies playground port — a
material-heavy procedural globe game that exposed startup stalls,
in-flight jank, and an import-time hang. None are game-specific; they
benefit any large OSS scene.

- featureFlags: enable shared `@import` scripts in OSS unconditionally.
  REACT_APP_SCRIPTS_ENABLED is unset everywhere, so every behavior that
  `@import`s a helper failed to import. Scripts are a first-class OSS
  authoring feature; integrated keeps the env opt-in.
- Editor / RenderEvent / RenderingQualityModule: make the scene's
  serialized `useShadows` the source of truth and seed the renderer's
  shadow tracking from it. A no-shadow scene was paying shadow-variant
  shader compiles + a full-scene shadow pass every frame, and the first
  post-(re)create frame spuriously marked every material needsUpdate.
  Adaptive quality may still downgrade shadows but never force them on.
- EngineRuntime: precompile material pipelines with compileAsync before
  the loop starts, and hold the loading overlay until the first frame
  renders — folding one-time shader compiles into play-start load
  instead of streaming jank in mid-play. Both best-effort/bounded.
- GameManager: init behaviors in small batches that yield to paint, so
  the loading overlay advances instead of the UI freezing during a
  large world build.
- BehaviorManager: drop the per-behavior fixedUpdate "not implemented"
  warning — dozens of warn-with-stacktrace calls stalled play start for
  no benefit; not implementing an optional hook is normal.
- Editor.runScriptImport: pause the editor render loop during a bulk
  script import so per-command cost stays flat (was ~O(n^2), a
  multi-minute hang that could OOM the renderer).
- remote-go thumbnail: guard the `/api/Scene/Edit` POST behind !IS_OSS;
  OSS has no such endpoint, so it 404-spammed on every import.

Adds the port planning doc under docs/planning/.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>