Add combat development sandboxes and fix rendering defensive code

[Copilot]

Problem

Created professional combat testing environments and hardened rendering code against missing data:

1. RenderSystem crash: drawEnemyResistanceIndicator accessed undefined baseResistances properties
2. Debug log spam: CollisionSystem flooded console with per-frame DEBUG logs
3. No combat sandbox: Needed isolated testing environment without waves/UI/audio

Changes

Defensive rendering (RenderSystem.js)

// Before: direct access crashes on missing fields
defense.shield.baseResistances[type]

// After: safe fallback chain
const layerData = defense[layerName];
const baseRes = layerData.baseResistances || layerData.resistances || {};
const bonusRes = layerData.bonusResistances || {};
const base = baseRes[type] || 0;
const bonus = bonusRes[type] || 0;

Debug log gating (CollisionSystem.js)

• Added COLLISION_DEBUG = false flag at file top
• Wrapped all 21 debug logs in if (COLLISION_DEBUG) conditionals
• Console clean by default, debugging available via flag toggle

Combat sandboxes (dev/)

Created three testing environments:

test-combat-sandbox.html - Manual system instantiation

• World + individual systems (no Game class)
• Full control over update loop and spawning

combat-sandbox-clean.html - Pure manual architecture

• No Game class, manual World/systems/player creation
• Explicit system.update() calls in custom loop

combat-sandbox.html - Game class override (recommended)

const game = new Game();
game.startGame();

// Override update to run only combat systems
game.update = function(deltaTime) {
    deltaTime = Math.min(deltaTime, 0.1);
    
    this.systems.movement.update(deltaTime);
    this.systems.ai.update(deltaTime);
    this.systems.combat.update(deltaTime);
    this.systems.defense.update(deltaTime);
    this.systems.collision.update(deltaTime);
    
    this.world.processPendingRemovals();
};

Benefits of override approach:

• Uses real Game systems/world/player (identical behavior to main game)
• No core file modifications
• WaveSystem/WeatherSystem/UISystem don't run (not called in override)
• Manual spawn controls via game.systems.spawner.spawnEnemy()

Supporting fixes

• Enemy firing cadence audit (already cooldown-gated, no changes needed)
• DefenseSystem wiring: world.defenseSystem = systems.defense
• GameState instantiation with minimal stats object
• Enemy spawn positions clamped to canvas bounds
• deltaTime clamping: Math.min(deltaTime, 0.1) prevents teleporting

Architecture decisions

Sandbox approach: Override game.update() rather than disable individual systems

• Cleaner: single override point vs nullifying multiple system methods
• Safer: systems remain intact, just not called
• Maintainable: clear what runs vs what doesn't

No core modifications: All sandbox logic contained in dev/*.html files

• Game.js, all systems, data files unchanged
• Main game (index.html) continues working identically
• Sandboxes can be deleted without affecting production code

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