docs: generic interaction system design

[lstep]

Summary

• Adds the complete design document for a generic, data-driven interaction system (documentation/plans/2026-07-15-interaction-system-design.md)
• Covers the full reasoning evolution (6 design decisions), final architecture, 5 worked examples, 4 sequence diagrams, data structures, extension handler patterns, and a 6-phase implementation plan
• Links to issue [Feature]: Generic interaction system (press E near entities → popup or immediate action) #129

Key design decisions

1. Two-phase RPC with immediate-mode opt-out — press E → query available actions → popup → choose → execute, OR immediate execution (entity's choice via on_interact_action property)
2. Entity data declares effects, extension code interprets them — interactions JSON property maps action IDs to effect lists (action verb + payload + target IDs)
3. Fully generic action strings — framework is a pure message router, never assumes a state model
4. Per-entity effects list — same action string ("toggle") can have different effects per entity; the data decides, not the code
5. Multi-extension dispatch — each extension handles verbs it knows, skips the rest; adding a new action type = one case in the extension, zero framework changes
6. Client-side sparks + overlay sprite glow — no server round-trip for cosmetic feedback

Test plan

• Document renders correctly on GitHub (markdown formatting, code blocks, tables)
• All referenced file paths and line numbers are accurate
• Sequence diagrams are readable in monospace rendering