Smart Hex protocol responsibilities

The Smart Hex participates as an addressed device on the shared main I²C bus. It reports local observations and accepts display and diagnostic requests. CHAIN provides enumeration eligibility separately from the bus data.

The canonical Hex ↔ Main protocol contains the enumeration proposal, candidate commands, conceptual framing, and unresolved error behavior. Keep wire definitions there so component descriptions cannot drift into a competing specification.

Hex-specific implementation responsibilities include bounded buffers, validating requests before acting, rendering semantic display commands locally, and reporting firmware identity. No packet byte values or completed firmware implementation are defined yet.