Redesign the interaction mechanic into a recursive chain reaction system
where items can trigger cascading reactions. Keys now open themed chests
which produce items that may trigger further reactions, with chain bonus
rewards for multi-step chains (x2/x3/x4+).
- Add 6 themed chests + mysterious chest + alchemist stone catalyst
- Rewrite InteractionEngine with recursive chain loop (max depth 10)
- Add ConsumeTrigger field to InteractionRule for catalyst support
- Add ChainBonusEvent and enrich InteractionTriggeredEvent with context
- Update rendering to show both reacting items and chain indicators
- Add item descriptions with anticipation hints for chain partners
- Update GDD Section 5 with full chain reaction specification
