Beta 23: reopen the last saved editor session

[WebCrew]

Implements the first of the two explicitly accepted final Phase 2 workspace follow-ups.

• reopens the last saved project scene or standalone scene after a normal application close
• restores project context, 2.5D/3D mode, bounded camera state and still-valid selection IDs
• loads actual saved scene data; unsaved edits are never embedded in session metadata
• preserves Save, Discard and Cancel semantics
• explicit Close Project/Delete Project clears the remembered context
• missing, moved, corrupt or incompatible files clear stale state and fall back to normal startup
• adds atomic, size-bounded local session storage and regression tests
• records Beta 22 human Windows acceptance and updates Beta 23 release/checklist material

Reference review: Godot exposes scene/session restoration settings, while Unity and Unreal preserve editor workspace/layout context. Parlyn keeps this first version deliberately smaller: one saved scene context with strict fallback and no coupling to project documents. Autosave remains a separate, independently tested Beta.

Local results: npm run check, release renderer build and git diff --check pass. Human Windows acceptance remains required after publication.