# Terrain Sculpt Tools (#23)

## Scope

- Add a modal terrain sculpt operator for raise, lower, flatten, smooth, and deterministic noise.
- Keep activation and brush settings in the existing horizontal viewport toolbar.
- Track a terrain-local cursor hit and render a world-space brush footprint preview.
- Apply live stroke previews while preserving one history entry per completed stroke.
- Restore the exact pre-stroke height grid on Escape or right-click.

## Implementation

1. Add pure heightfield sampling, ray-hit, falloff, and brush-dab functions with focused tests.
2. Add editor-only terrain sculpt state and viewport input/preview systems.
3. Commit strokes through the reflected component transaction after restoring the pre-stroke snapshot.
4. Add compact terrain controls to the existing viewport toolbar and update operator hints.
5. Document controls and validation evidence in the terrain editor guide and root checklist.

## Acceptance

- All five modes visibly modify terrain.
- The brush ring follows the actual terrain surface.
- Escape and right-click cancel without dirtying history.
- Mouse release creates exactly one undoable transaction regardless of dab count.
- Formatting, editor/shared checks, focused tests, and live editor QA pass.
