fix(wall-tool): snap length to grid instead of absolute coords to fix 90° gap at off-grid points

[zephran-dev]

Problem

When starting a wall from an off-grid endpoint (e.g. 17ft = 5.18m), the
angle snap would correctly lock the direction to 90°, but then
snapPointToGrid would round the absolute X/Z coordinates — moving the
endpoint away from the true perpendicular and creating micro-gaps in
closed rooms.

Root Cause

In snapPointTo45Degrees, after snapping the angle the result was passed
through snapPointToGrid, which rounds world-space coordinates to the
global grid regardless of where the start point is.

Fix

Instead of snapping the final XZ position to the grid, snap only the
distance (scalar) to the grid step. The direction stays locked to the
snapped angle and the start point is preserved exactly.

Closes #256