[squad:plan] Echo Bloom — Vertical Slice

**Owner:** Fenster (Engineering/Systems), with Hockney (QA) support

Milestone deliverables:
- Build 1 polished biome with final-look lighting, UI, and layered audio system
- Expand to 4 plant types and pollinator creature behavior
- Deliver 3 complete chambers demonstrating progression and mechanic escalation

Part of the Echo Bloom game plan (#54786).

Start with the Echo Bloom game plan in issue #54786 and review how this vertical slice fits its milestone deliverables. The work covers a polished biome, four plant types, pollinator behavior, layered audio, UI, lighting, and three progression chambers; done means all listed deliverables are complete.

Written by the indexing model from the issue text.