Beta 20: preserve saved Undo/Redo per project scene

[WebCrew]

Defect and cause

Maintainer Beta 19 testing found saved Undo disappearing after A/B scene switches.
All project scene saves used one startup-scene history file; saving B overwrote A.
The existing mismatch protection then correctly rejected B's history when A reopened.

Fix

• separate SHA-256-keyed history files per validated project-relative scene path
• preserve snapshot/path matching, atomic writes, format validation and size limits
• use guarded parent creation and retain matching read-only legacy history fallback
• never fall back from corrupt primary history to an older legacy file
• wire project-open, scene-open and scene-save through the shared file module

Scope and verification

Bug fix only; no new roadmap functionality. Beta 19 remains unaccepted.
Beta 20 version, release notes, README, roadmap, format docs and checklist updated.
Local npm run check, renderer release build and git diff --check passed.
New real-filesystem tests cover two saved scenes, same-name scenes in separate
folders, saved Undo/Redo, stale/corrupt rejection and read-only legacy fallback.
The local Electron runtime download was previously unavailable; required Windows
packaging runs its desktop smoke check.
Already overwritten Beta 19 histories cannot be reconstructed.
CI, CodeQL and Windows build must pass; human Beta 20 re-test remains required.