feat(projectile): add reusable projectile runtime for towers and future weapons

[f1cklepickle]

Why

• The first tower should fire real projectiles so the combat path is reusable for future ranged content.
• A reusable projectile system will also open a path for later crossbow-style player weapons without duplicating combat code.

Proposed Change

• Add a reusable projectile prefab/runtime with travel, hit, damage, and cleanup behavior.
• Define ownership/target filtering so tower projectiles only affect valid enemies.
• Keep projectile behavior modular so future ranged weapons can reuse the same system.

Acceptance Criteria

• Towers can launch projectiles that travel, hit valid enemies, apply damage, and clean up correctly.
• The projectile runtime is reusable beyond the first tower implementation.

Testing / Validation

• Manual:
  • Verify tower-fired projectiles travel toward enemies and resolve damage on hit.
• Automated:
  • EditMode: Projectile hit/filtering tests
  • PlayMode: Projectile travel/hit smoke test