feat(tower): add tower prefab and core runtime

[f1cklepickle]

Why

• Barrier tower construction needs a real tower object with clean runtime ownership before build options can stay modular.
• The first tower should establish reusable structure for stats/state, health, and future variation without coupling targeting and attack logic into one script.

Proposed Change

• Add a tower prefab with core runtime components and serialized references.
• Include tower identity, basic stats/state, and a health field/component for future ranged-enemy interactions.
• Keep targeting and attack execution delegated to separate components.

Acceptance Criteria

• A reusable tower prefab exists and can be spawned into the scene.
• Tower runtime exposes health/state cleanly without requiring targeting or attack logic to live in the same component.

Testing / Validation

• Manual:
  • Spawn the tower prefab in a test scene and verify it initializes correctly with expected references/state.
• Automated:
  • EditMode: Tower prefab/runtime contract tests
  • PlayMode: Basic tower spawn smoke test