feat(barrier): add tower plots and build integration to upgrade menu

[f1cklepickle]

Why

• Barrier upgrades need to become more expandable than a single health option.
• Authored tower plots tied to barriers will make pre-wave build choices clearer and support the first meaningful new defense option.

Proposed Change

• Extend the existing barrier upgrade menu to support multiple options cleanly.
• Add authored tower plot components linked to barriers for valid build locations.
• Allow the player to spend gold to build a tower from the upgrade menu onto an empty linked plot.

Acceptance Criteria

• Barrier upgrade UI can present at least health upgrade and build-tower options cleanly.
• The player can spend gold to build a tower on a valid authored plot linked to a barrier.
• Occupied plots can no longer be purchased again.

Testing / Validation

• Manual:
  • Open the between-wave upgrade flow, select a barrier, and verify tower purchase/build works on a valid plot.
• Automated:
  • EditMode: Barrier tower-plot/menu contract tests
  • PlayMode: Tower build purchase flow smoke test