Beta 19: optional scene and world authoring bounds

[WebCrew]

Scope

Implements the next Phase 2 roadmap item as optional world-space authoring guides.

• validated independent scene/world minimum and maximum coordinates
• blue scene and amber world bounds via the View group's Bounds dialog
• scene Undo/Redo and normal saving; explicit world save with failure-safe adoption
• bounds helpers excluded from node selection and Frame Selected
• no physics walls, camera clamps, streaming, or Smart Systems feature expansion

Compatibility and release

Existing scene-v2/world-v1 files without bounds retain their serialized shape.
Older editors may discard optional bounds on save; this is documented.
Updates version to Beta 19, README, roadmap, format docs, testing checklist and
unsigned release workflow notes/portable ZIP names.
Beta 18 passed the maintainer's full regression test; Beta 19 human acceptance is pending.

Local verification

• npm run check: passed, including bounds validation, document/history roundtrips,
  Undo/Redo and renderer helper lifecycle checks
• npm run build:renderer:release: passed
• git diff --check: passed
• local Electron runtime remains unavailable after its previously stalled download;
  Windows CI executes the desktop smoke test during the required packaging build

CI, CodeQL and Windows build must all pass before merge.
Portable Beta 19 will be human-tested before this item is fully accepted.