Add chicken entities with harvestable meat as food source

[Copilot]

Implements wandering chicken NPCs that drop meat when harvested, providing an additional food source beyond apples.

Implementation

Entity System

• Added CHICKEN to EntityType enum and CHICKEN_MEAT to ItemType enum
• Created Chicken entity class with autonomous wandering behavior (retargets every 3s, speed 5-10 units)
• Chickens render as 0.5x scale capsules in beige, drop 1-2 meat on harvest

World Generation

• 20% chunk spawn probability, 1-3 chickens per spawn
• Lag prevention cap at 50 total chickens

Game Loop Integration

• Chickens update via wander() in WorldScreen.FixedUpdate
• Uses cached Rigidbody reference and Time.fixedDeltaTime for consistent physics

Economy

• Chicken meat base cost: 50 coins (between apples at 40 and wood at 60)
• Fully integrated with existing inventory and market systems
• UI updated to display chicken meat count

// Chicken harvesting added to InteractCommand
else if (chicken != null && Vector3.Distance(player.getPosition(), chicken.getPosition()) < 5) {
    chicken.markForDeletion();
    player.getInventory().transferContentsOfInventory(chicken.getInventory());
    player.getStatus().update("Harvested chicken meat.");
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>Chickens</issue_title>
<issue_description>We should have chickens wandering around that can be utilized as a source of food.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Chickens #83

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