I2C/hardware sensor support (scope addition beyond the MQTT proposal)

[sidick]

Add I2C/GPIO-connected external sensor support to midge, once the current phased roadmap (protocol core → mqtt.library → TLS → dashboard → release) is further along.

This is a scope addition beyond the original proposal (project-ideas/pending/amimqtt-proposal.md), whose only "sensor" data was the Amiga's own synthetic telemetry (uptime, RAM free, CPU model) published in Phase 4. Real I2C hardware sensors are separate.

Test harness: copperline-plugin-cpldicy paired with Copperline — a function-complete emulation of the real Zorro CPLDicy I2C interface card, supporting custom user-configured I2C accessories rather than a fixed set. Analogous to how [hostsocket] let Phase 1's networking tests run without real hardware. Check that plugin repo's current API/capabilities before designing the transport.

Design direction: plan as its own phase/slice — likely another transport-style seam in src/core or a new peripheral abstraction, following the same portable-core/platform-glue split already used for MQTT transports — rather than folding it into the existing MQTT phases.

[sidick]

The telemetry half of this (Amiga's own synthetic sensors - uptime, chip/fast RAM free, CPU model - published to Home Assistant via MQTT Discovery) is now implemented as a standalone headless Commodity, mqttstats, in #22: configured entirely via Workbench ToolTypes, runnable silently from WBStartup, Commodities Exchange as its only control surface.

I2C/external hardware sensor support (this issue's actual scope) is still to come as a follow-up on top of that - deliberately deferred so the commodity's plumbing (config, MQTT Discovery, WBStartup-safe silent operation) lands first.