fix(engine): reduce ZoneMinRadius from 5 to 3 to force combat engagement

The previous ZoneMinRadius=5 created a final zone diameter of 10 tiles,
which allowed bots to remain outside the 3.5-tile attack radius even when
both were inside the zone. This resulted in low combat_death rates for
passive bot strategies (~10% for random bots vs the 65-80% target).

With ZoneMinRadius=3, the final zone diameter is 6 tiles, forcing bots
into proximity where focus-fire combat triggers more consistently.

Also adds verify-combat-density.sh script for ongoing metrics tracking.

Closes: bf-4bj9

docs/plan/plan.md

@@ -488,7 +488,7 @@ rather than pure energy farming. Zone parameters are tuned per player count:
 | ZoneStartTurn | 20 | 15 | Turn when zone begins shrinking |
 | ZoneShrinkInterval | 2 | 2 | Turns between shrink steps |
 | ZoneShrinkStep | 2 | 2 | Tiles to shrink each step |
-| ZoneMinRadius | 5 | 5 | Minimum zone radius (stops shrinking) |
+| ZoneMinRadius | 3 | 3 | Minimum zone radius (stops shrinking) |
 
 **Design rationale:**
 - **ZoneStartTurn**: Starts early enough to force combat before energy farming dominates,
@@ -498,8 +498,8 @@ rather than pure energy farming. Zone parameters are tuned per player count:
   being too chaotic.
 - **ZoneShrinkStep = 2**: 2 tiles per interval is aggressive enough to force engagement
   while allowing time for tactical movement.
-- **ZoneMinRadius = 5**: Small enough that bots within the zone are in attack range
-  (~3.5 tiles), but large enough to avoid killing bots that spawn near the edge.
+- **ZoneMinRadius = 3**: Final zone diameter (6 tiles) forces bots within attack range
+  (~3.5 tiles), ensuring combat occurs before the zone kills everyone.
 
 **Combat density metrics** (verified with local testing):
 - 2-player: ~65-80% of matches have combat_deaths; ~1 death per 20 turns

engine/types.go

@@ -243,13 +243,13 @@ func ConfigForPlayers(numPlayers, coresPerPlayer int) Config {
 		cfg.ZoneStartTurn = 20     // Start zone early to force combat before farming wins
 		cfg.ZoneShrinkInterval = 2 // Shrink every 2 turns (faster pressure)
 		cfg.ZoneShrinkStep = 2     // Shrink 2 tiles per interval
-		cfg.ZoneMinRadius = 5      // >= spawn radius (4 tiles) ensures bots survive to final zone
+		cfg.ZoneMinRadius = 3      // Final zone diameter (6) is closer to attack radius (3.5)
 		cfg.AttackRadius2 = 12     // 3.5 tiles (balanced for 2-player)
 	} else {
 		cfg.ZoneStartTurn = 15     // Start zone early for 3+ players (larger gap to close)
 		cfg.ZoneShrinkInterval = 2 // Shrink every 2 turns (faster pressure)
 		cfg.ZoneShrinkStep = 2     // Shrink 2 tiles per interval
-		cfg.ZoneMinRadius = 5      // >= spawn radius (5 tiles) ensures bots survive to final zone
+		cfg.ZoneMinRadius = 3      // Final zone diameter (6) forces bots into attack range (3.5)
 		cfg.AttackRadius2 = 12     // 3.5 tiles (same as 2-player for better combat trigger)
 	}
 

scripts/verify-combat-density.sh

@@ -0,0 +1,55 @@
+#!/bin/bash
+# Verify combat_death event rates across different bot combinations
+# Expected: 2-player ~65-80%, 6-player 100%
+
+set -e
+
+cd "$(dirname "$0")/.."
+
+echo "=== Combat Density Verification ==="
+echo "Running 20 matches per configuration..."
+echo
+
+# Function to run matches and calculate combat_death rate
+run_matches() {
+    local bots="$1"
+    local count="$2"
+    local description="$3"
+    local with_deaths=0
+    local total_deaths=0
+
+    echo "Testing: $description"
+    for i in $(seq 1 $count); do
+        local output="/tmp/verify-$i.json"
+        ./acb-local -bots $bots -max-turns 100 -output $output >/dev/null 2>&1
+        local deaths=$(python3 -c "import json; r=json.load(open('$output')); print(len([e for t in r['turns'] for e in t.get('events', []) if e.get('type') == 'combat_death']))" 2>/dev/null || echo 0)
+        if [ "$deaths" -gt 0 ]; then
+            with_deaths=$((with_deaths + 1))
+            total_deaths=$((total_deaths + deaths))
+        fi
+    done
+
+    local rate=$((with_deaths * 100 / count))
+    echo "  Matches with combat_deaths: $with_deaths/$count ($rate%)"
+    echo "  Total combat_death events: $total_deaths"
+    echo "  Average per match: $(python3 -c "print($total_deaths / $count)")"
+    echo
+
+    # Return 1 if rate is below threshold
+    if [ "$description" = "2-player" ] && [ $rate -lt 65 ]; then
+        return 1
+    fi
+    if [ "$description" = "6-player" ] && [ $rate -lt 100 ]; then
+        return 1
+    fi
+    return 0
+}
+
+# Test 2-player matches
+run_matches "random,random" 20 "2-player (random bots)"
+run_matches "gatherer,rusher" 20 "2-player (aggressive bots)"
+
+# Test 6-player matches
+run_matches "random,gatherer,rusher,guardian,swarm,hunter" 20 "6-player (mixed bots)"
+
+echo "=== Verification Complete ==="