Add Ground Placement command and publish Beta 15

[WebCrew]

Summary

• add a Ground toolbar action and the End keyboard shortcut
• place selected objects using their actual rendered bounds against the editor ground
• support multi-selection as one Undo/Redo transaction
• preserve scene saving, dirty-state handling, snapping, and Local/World gizmo behavior
• document the Beta 15 acceptance test and update the roadmap

Automatic surface-aware placement remains a later, deliberately separate roadmap step.

Verification

• node scripts/check.cjs
• git diff --check
• exact GitHub comparison: 1 commit, 15 intended files, 86 additions, 22 deletions