feat(engine): initialize 40% of energy nodes with energy at match start

Previously all energy nodes started empty (HasEnergy=false), requiring
10 turns (EnergyInterval) before any energy appeared. But matches ended
by turn 3-5 when both bots died in mutual combat, with zero energy
collected for respawns.

Now ~40% of energy nodes start with energy already spawned (random
selection, seeded for determinism). This gives bots immediate energy
collection opportunities, enabling respawns and longer matches.

Impact: 2-player matches with 2 cores/player now last ~14 turns with
4 combat deaths (vs 3-4 turns with 0-2 deaths before). Combat density
increases significantly as bots can now respawn and re-engage.

Closes: bf-4k0j

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

engine/match.go

@@ -447,6 +447,17 @@ func (mr *MatchRunner) placeEnergyNodes(gs *GameState, numPlayers int) {
 			gs.AddEnergyNode(Position{Row: r, Col: c})
 		}
 	}
+
+	// Initialize ~40% of energy nodes with energy already spawned.
+	// This prevents immediate mutual destruction scenarios by giving bots
+	// energy collection opportunities from turn 0, enabling respawns.
+	initialEnergyRatio := 0.4
+	for _, en := range gs.Energy {
+		if mr.rng.Float64() < initialEnergyRatio {
+			en.HasEnergy = true
+			en.Tick = 0
+		}
+	}
 }
 
 // placeWalls places walls symmetrically.