Ilia Denisov
d63fe44618
The pre-existing `Deltas` helper used the height to wrap the x-axis, which silently produced wrong values on any rectangular galaxy (`w != h`). Square galaxies — the only configuration the engine ships today — masked the bug, so it stayed in tree. `Deltas` is now a thin wrapper around the new `ShortestDelta(a, b, size)`, which returns the signed per-axis shortest delta on a 1-D circle (range `(-size/2, size/2]`). The signed flavour is what the Phase 19 ship-group renderer needs to draw an IncomingGroup trajectory across the torus seam; `Deltas` continues to return the pair of absolute deltas for distance computation. Adds `pkg/calc/map_test.go` with table-driven coverage for both helpers, including a regression that exercises the rectangular case the bug was hiding behind, and the half-circumference tie-break. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>