Beta 21: optional surface placement on Move release

[WebCrew]

Roadmap scope

Initial optional automatic surface placement in active Phase 2.

• persistent, default-off Surface toggle with own SVG, tooltip and pressed state
• on changed Move release only, place the primary mesh/sprite/billboard vertically
  on the nearest suitable lower authored Mesh3D face or existing editor ground
• one Undo transaction for movement and placement
• exclude self, selected/related nodes, disabled and unsuitable faces
• bounded downward ray; no upward penetration resolution, slope rotation,
  collision/physics, imported terrain, Inspector snapping or multi-node Move expansion
• Rotate/Scale, Ground/End, Transform Snap and bounds/history remain separate

Planning and acceptance

Beta 20 human acceptance is recorded.
Document the explicitly approved later Reopen Last Session and autosave-draft
steps in Phase 2, after placement and command checks; not implemented here.
Beta 21 version, README, roadmap, checklist and portable release notes updated.

Verification

Local npm run check, renderer release build and git diff --check passed.
New tests cover nearest support, fallback, overhead/disabled exclusion,
billboard placement, no-op eligibility, actual editor commit opt-out and
Rotate/Scale independence, and one-step Undo.
The local Electron runtime download was previously unavailable; required Windows
packaging runs the desktop smoke test.
CI, CodeQL and Windows build must pass before merge.
Beta 21 remains pending human portable Windows acceptance.