v4.5.7

Activity tutorials now start after the editor finishes loading its assets, project, stage and runtime. This fixes a startup race where slow loading left Cog or Marty tutorials without instructions and controls.

Tutorial startup is restricted to the editor and runs once, preserving progress when readiness callbacks repeat. This release also includes the existing change that opens micro:bit setup when its extension is added.

Validation: 110 unit tests; eight browser E2E checks covering delayed assets in direct and embedded routes, tutorial save/reopen, library return navigation, and saved-project hard refresh; production build; and companion wrapper tests (167), typecheck, lint and iOS/Android bundles. The delayed-asset regression fails against the original implementation and passes with this fix. Physical iPad Air 2/iPadOS 15 verification remains outstanding.