Reduce rail signal-state churn in the simulation hot path

[A-BenLi06]

Summary

Reduce per-rail allocation and copying in the once-per-second simulation hot path without changing signal reservation semantics.

Problem

For every rail and every simulation tick, the current implementation:

• creates a separate client-array snapshot;
• compares reservation key sets through generic collection helpers;
• clears and copies four AVL maps;
• uses a stream to test blocked vehicle IDs.

The same client set is therefore materialized R times for R rails, and unchanged reservation maps still incur copying.

Change

• Snapshot clients once in Simulator.tick() and pass the array to every rail.
• Rotate current/previous reservation maps instead of copying their contents.
• Track key-set changes while reservations are written and retain size checks for removals.
• Replace the boxed/streamed value scan with a primitive iterator.
• Add a regression test proving reservation state survives map rotation.

The unavoidable client update scan remains O(R × C), but temporary client arrays fall from O(R × C) references to O(C), and reservation history rotation becomes O(1) rather than O(entries).

Validation

• ./gradlew test --no-daemon
• Full suite passed (113 tests).

Part of #32.