Add configurable active zone that contracts toward map center on an interval,
forcing bots together to trigger focus-fire engagements. Bots outside the zone
die with reason "zone", and zone bounds are recorded in replay turns.
Config fields:
- ZoneEnabled: enable/disable the zone
- ZoneStartTurn: turn when zone starts shrinking (default 50)
- ZoneShrinkInterval: turns between shrink steps (default 5)
- ZoneShrinkStep: tiles to shrink each step (default 2)
- ZoneMinRadius: minimum zone radius (default 10)
Turn sequence update: MOVE → COMBAT → ZONE → CAPTURE → COLLECT → SPAWN → ENERGY_TICK
Zone phase inserted after COMBAT, uses toroidal distance calculation,
and emits bot_died events with reason "zone" for killed bots.
Replay update: ZoneBounds struct added to ReplayTurn to record center,
radius, and active state per turn for visualization.
Determinism verified: all tests pass, engine remains deterministic.
Closes: bf-2g96
16d474aceb