Parent: #131. Depends on completed guided canvas #130.
Add session-only view controls that never paint or mutate the artwork.
Acceptance:
- The artist can hide and show guide numbers at any coverage.
- The artist can temporarily preview the generated target colors over the board.
- Target preview never writes pixels, changes history, or fills remaining cells.
- View choices are reset safely when guidance starts, reloads, or finishes.
- Pointer input still reaches the drawing canvas and CRT state is untouched.
Parent: #131. Depends on completed guided canvas #130.
Add session-only view controls that never paint or mutate the artwork.
Acceptance: