Direction-based lane offset for train markers

[Jorioz]

What to build

When two trains are close together it is hard to tell them apart or read their directions: the current overlap adjustment only reacts once markers collide, shrinking both to half size and nudging them by a small perpendicular offset, which leaves them mostly stacked.

Replace this with a structural rule: every train marker always rides a consistent side of its line based on the train's direction, like double-track railways (e.g. inbound on one side, outbound on the other, consistently across the whole map). Opposite-direction trains then never overlap by construction, and direction becomes readable spatially at a glance. Keep a much milder residual adjustment (small stagger, no aggressive shrink) for same-direction bunching only.

Acceptance criteria

• A moving train renders offset to the side of its line determined by its direction, consistently across all lines and segments (including line extensions)
• Two trains passing in opposite directions render as two clearly separate markers on opposite sides of the line
• Same-direction trains that bunch up remain individually distinguishable without shrinking to illegibility
• Markers still track the line correctly through corners/bends at the offset
• Selection highlight, dimming, and click targets keep working at the offset position

Blocked by

None - can start immediately