Add Transform Snap foundation and publish Beta 12

[WebCrew]

Summary

• add a persistent Transform Snap toggle to the viewport toolbar
• use fixed initial increments: 0.5 units for Move, 15 degrees for Rotate and 0.1 for Scale
• apply snapping through THREE TransformControls while preserving existing 2.5D axis constraints and Undo/Redo
• keep Inspector entry continuous and independent from gizmo snapping
• add validated settings, automated contracts, roadmap status and a focused Beta 12 acceptance guide

Verification

• node scripts/check.cjs
• git diff --check
• exact GitHub branch comparison against the Beta 11 main commit

The local bundled Electron smoke test was not rerun because the temporary dependency directory was no longer present. Parlyn CI and the Windows Installer workflow must pass before merge.