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>
Ilia Denisov