Place inserted pieces under the cursor and aligned to studs. Fixes #1020.

[roor0]

Summary

Fixes #1020 — inserting an odd-sized part (1x1, 1x3, …) onto an even-sized part (2x2, 1x2, …) landed it half a stud off, between studs.

Root cause

When inserting a piece onto an existing piece, lcView::GetMouseInsertPieceInfo took the position from the hit-face vector (ObjectRayTest.PieceInfoRayTest.Plane), which is zero on the two in-face axes — it only identifies which face was hit. The new piece was therefore always placed at the target piece's origin, ignoring where the cursor actually was.

Because a piece's local origin sits on a stud for odd stud counts but between studs for even counts, an odd-sized part dropped onto an even-sized part landed half a stud off. Same-parity pairs (odd-on-odd, even-on-even) happened to line up, which is why other bricks seemed fine.

Fix

• Use the actual cursor hit point on the surface for the two in-face axes (instead of the target origin), so pieces land where you point.
• Snap those axes to the stud grid with a half-stud phase when the inserted and target pieces have different stud-count parity on that axis, so studs line up on studs.
• A finer (half-stud) snap still reaches the positions between studs — e.g. placing a round 1x1 in the centre of a 2x2 — so no placement is lost.
• The hit-face axis keeps the existing "abut against the face" offset, and vertical stacking rounding is unchanged.

Testing

• Builds cleanly (Qt 6.6.3, macOS) and verified in the app.
• Checked 1x1 / 1x3 / round-1x1 onto 2x2, 1x2, and 1x1 targets, at both 1-stud and half-stud move snap. At 1-stud snap pieces land on studs; at half-stud snap the between-stud positions remain reachable. Vertical stacking is unchanged.