docs(plan): align zone parameters with proven implementation

Updated plan §3.7.1 zone parameter table to reflect the proven
implementation in ConfigForPlayers(). The previous plan values
(ZoneStartTurn=20/15, ZoneShrinkInterval=2) did not achieve the
stated combat density targets. The current values
(ZoneStartTurn=10, ZoneShrinkInterval=1) achieve 77% combat density
for 2-player and 99% for 6-player (targets: 65-80% and 100%).

Also updated code comments in engine/types.go to remove outdated
references to the old plan values.

TestCombatDensityMetrics passes with these parameters.

Closes: bf-3og6

docs/plan/plan.md

@@ -492,17 +492,17 @@ rather than pure energy farming. Zone parameters are tuned per player count:
 
 | Parameter | 2-Player | 3+ Player | Description |
 |-----------|----------|-----------|-------------|
-| ZoneStartTurn | 20 | 15 | Turn when zone begins shrinking |
-| ZoneShrinkInterval | 2 | 2 | Turns between shrink steps |
+| ZoneStartTurn | 10 | 10 | Turn when zone begins shrinking |
+| ZoneShrinkInterval | 1 | 1 | Turns between shrink steps |
 | ZoneShrinkStep | 2 | 2 | Tiles to shrink each step |
 | ZoneMinRadius | 3 | 1 | Minimum zone radius (stops shrinking) |
 
 **Design rationale:**
-- **ZoneStartTurn**: Starts early enough to force combat before energy farming dominates,
-  but late enough to allow early-game positioning. 2-player gets 5 extra turns due to
-  smaller maps (faster engagement).
-- **ZoneShrinkInterval = 2**: Shrinks every 2 turns creates steady pressure without
-  being too chaotic.
+- **ZoneStartTurn = 10**: Starts early to force combat before energy farming dominates.
+  Both 2-player and 3+ use the same start turn for consistent forcing function timing.
+- **ZoneShrinkInterval = 1**: Shrinks every turn creates steady, predictable pressure.
+  Faster than the original 2-turn interval to ensure bots reach contact range before
+  the match is decided by energy alone.
 - **ZoneShrinkStep = 2**: 2 tiles per interval is aggressive enough to force engagement
   while allowing time for tactical movement.
 - **ZoneMinRadius = 3 (2-player)**: Final zone diameter (6 tiles) forces bots within attack

engine/types.go

@@ -238,16 +238,16 @@ func ConfigForPlayers(numPlayers, coresPerPlayer int) Config {
 	// Scale zone parameters to force combat contact
 	// Zone must start early to force combat before energy farming wins
 	// ZoneMinRadius must be >= spawn radius so bots aren't killed before they can reach attack range
-	// Plan §3.7.1 targets (65-80% combat for 2-player, 100% for 6-player) require more aggressive zone
+	// Per plan §3.7.1
 	if numPlayers == 2 {
-		cfg.ZoneStartTurn = 10     // Earlier start to force combat before energy farming dominates
-		cfg.ZoneShrinkInterval = 1 // Shrink every turn for faster engagement (plan says 2)
+		cfg.ZoneStartTurn = 10     // Per plan §3.7.1
+		cfg.ZoneShrinkInterval = 1 // Per plan §3.7.1
 		cfg.ZoneShrinkStep = 2     // 2 tiles per interval (per plan §3.7.1)
 		cfg.ZoneMinRadius = 3      // Final zone diameter (6) forces bots into attack range (6)
 		cfg.AttackRadius2 = 36     // 6 tiles per plan §3.4 (2-player)
 	} else {
-		cfg.ZoneStartTurn = 10     // Earlier start for 3+ players (plan says 15)
-		cfg.ZoneShrinkInterval = 1 // Shrink every turn for faster engagement (plan says 2)
+		cfg.ZoneStartTurn = 10     // Per plan §3.7.1
+		cfg.ZoneShrinkInterval = 1 // Per plan §3.7.1
 		cfg.ZoneShrinkStep = 2     // 2 tiles per interval (per plan §3.7.1)
 		cfg.ZoneMinRadius = 1      // Zone diameter (2) < attack radius (3.5), forces contact
 		cfg.AttackRadius2 = 12     // 3.5 tiles per plan §3.4 (3+ player)