Correct bank tank bounds on bridge > 7

Road tanks at bridge terrain inherit MinX=8/MaxX=238 (full
screen with boundaryPadding) because renderBandedLines stores
LeftX=0/RightX=ScreenWidth for every bridge scanline. After
bridge destruction on levels > 7 the road tank was converted to
bank-tank but kept those wide bounds, so terrainAhead stayed true
across the whole screen and the tank walked over the gap instead
of stopping at its edge.

Introduce convertToBankTank which sets TankLocation and derives
MinX/MaxX from the gap constants so the converted tank is bounded
to its bank. Expand the existing test to pre-set road-tank bounds
and verify MinX/MaxX after conversion; add a right-bank case.

Author: morozov

pkg/logic/collision.go

@@ -198,7 +198,7 @@ func (b bridgeTarget) onHit(r *CollisionResult) {
 			r.PointsScored += PointsTank
 		} else if b.bridgeIndex > bridgeEarlyLevel {
 			// Tank is on the bank, late level: convert to bank-tank behavior.
-			obj.TankLocation = domain.TankLocationBank
+			convertToBankTank(obj)
 		}
 		// Early level: tank remains frozen (moveTank skips road tanks
 		// while bridgeDestroyed is true).

pkg/logic/collision_test.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/morozov/river-raid-ebiten/pkg/assets"
 	"github.com/morozov/river-raid-ebiten/pkg/domain"
+	"github.com/morozov/river-raid-ebiten/pkg/platform"
 	"github.com/morozov/river-raid-ebiten/pkg/state"
 )
 
@@ -511,15 +512,18 @@ func TestApplyBridgeDestroyedTanks_InGap(t *testing.T) {
 	}
 }
 
-func TestApplyBridgeDestroyedTanks_OnBank_LateLevel(t *testing.T) {
+func TestApplyBridgeDestroyedTanks_OnBank_LateLevel_LeftBank(t *testing.T) {
 	t.Parallel()
 
+	// Tank is left of the gap (X=0x20, X+spriteWidth=0x2A < 0x70).
 	vp := state.NewViewport()
 	obj := &state.ViewportObject{
 		X: 0x20, Y: 50,
 		Type:         domain.ObjectTank,
 		TankLocation: domain.TankLocationRoad,
 		Activated:    true,
+		MinX:         8,   // road-tank bounds (wide, full screen)
+		MaxX:         238, // road-tank bounds
 	}
 	vp.Objects = append(vp.Objects, obj)
 
@@ -532,6 +536,47 @@ func TestApplyBridgeDestroyedTanks_OnBank_LateLevel(t *testing.T) {
 	if obj.TankLocation != domain.TankLocationBank {
 		t.Errorf("TankLocation = %v, want TankLocationBank", obj.TankLocation)
 	}
+	if obj.MinX != 0 {
+		t.Errorf("MinX = %d, want 0", obj.MinX)
+	}
+	wantMaxX := tankGapLeftEdge - assets.SpriteTankWidth - boundaryPadding
+	if obj.MaxX != wantMaxX {
+		t.Errorf("MaxX = %d, want %d (gap left edge minus sprite width minus padding)", obj.MaxX, wantMaxX)
+	}
+}
+
+func TestApplyBridgeDestroyedTanks_OnBank_LateLevel_RightBank(t *testing.T) {
+	t.Parallel()
+
+	// Tank is right of the gap (X=0xA0 > 0x90).
+	vp := state.NewViewport()
+	obj := &state.ViewportObject{
+		X: 0xA0, Y: 50,
+		Type:         domain.ObjectTank,
+		TankLocation: domain.TankLocationRoad,
+		Activated:    true,
+		MinX:         8,   // road-tank bounds (wide, full screen)
+		MaxX:         238, // road-tank bounds
+	}
+	vp.Objects = append(vp.Objects, obj)
+
+	var result CollisionResult
+	bridgeTarget{vp: vp, bridgeIndex: bridgeEarlyLevel + 1}.onHit(&result)
+
+	if len(result.DestroyObjects) != 0 {
+		t.Errorf("DestroyObjects = %v, want empty (tank should become bank-tank)", result.DestroyObjects)
+	}
+	if obj.TankLocation != domain.TankLocationBank {
+		t.Errorf("TankLocation = %v, want TankLocationBank", obj.TankLocation)
+	}
+	wantMinX := tankGapRightEdge + boundaryPadding
+	if obj.MinX != wantMinX {
+		t.Errorf("MinX = %d, want %d (gap right edge plus padding)", obj.MinX, wantMinX)
+	}
+	wantMaxX := platform.ScreenWidth - assets.SpriteTankWidth
+	if obj.MaxX != wantMaxX {
+		t.Errorf("MaxX = %d, want %d (screen width minus sprite width)", obj.MaxX, wantMaxX)
+	}
 }
 
 func TestApplyBridgeDestroyedTanks_OnBank_EarlyLevel(t *testing.T) {

pkg/logic/enemy_ai.go

@@ -65,6 +65,21 @@ func initializeObjectBoundaries(obj *state.ViewportObject, terrain TerrainBuffer
 	}
 }
 
+// convertToBankTank switches a road tank to bank-tank behavior after bridge destruction
+// on a late level (spec §7.5.4). Bounds are derived from the bridge gap position so the
+// tank cannot cross the gap — it will stop at the bank edge and fire.
+func convertToBankTank(obj *state.ViewportObject) {
+	obj.TankLocation = domain.TankLocationBank
+	sprite := assets.SpriteObjects[domain.ObjectTank]
+	if obj.X+tankGapProbe < tankGapLeftEdge {
+		obj.MinX = 0
+		obj.MaxX = tankGapLeftEdge - sprite.Width - boundaryPadding
+	} else {
+		obj.MinX = tankGapRightEdge + boundaryPadding
+		obj.MaxX = platform.ScreenWidth - sprite.Width
+	}
+}
+
 // moveEnemies updates all activated enemy positions based on their type-specific AI.
 // gameplayMode is used to suppress helicopter missile firing during scroll-in.
 // bridgeDestroyed freezes road tank movement when true.