Homie 5 compliance: convention-compliant base layer, eBus policy above it #91

Goal: python-sdk's base Homie layer (ebus_sdk.homie: Device, Node, Property, Controller) is compliant with the Homie 5 convention on the wire and in its base semantics. eBus and SDK policy that goes beyond the convention lives in clearly labeled higher-level methods, documented as such.

An audit of homie.py against the convention (2026-10-08, every finding independently verified) found about 35 distinct issues. Most are base-layer defects visible to any Homie 5 controller, not deliberate extensions. The phases below group them; each phase issue lists its items with convention citations.

Policies for this work:

• Invalid producer input (ids, missing required $format, null description fields): warn in the next release, raise in the minor release after it.
• $description version: a content hash (an integer below 2^53, SHA-256 of the description excluding version). The convention requires a new version whenever the document changes and allows non-sequential values (convention.md:215). An unchanged description keeps its version across reconnects.
• Effective state: the base Controller implements the convention's table (only a lost root overrides a child, convention.md:262-267). The wider cascade that span-hass and esp32-sdk rely on stays available under its current name, documented as an availability rule beyond Homie, and is proposed to the eBus specification.

Phases:

• Phase 1: producer wire correctness (Homie compliance phase 1: producer wire correctness #95, done in Homie 5 compliance, phase 1: producer wire correctness #96)
• Phase 2: id, topic and domain validation (Homie compliance phase 2: id, topic and domain validation #92)
• Phase 3: Controller robustness (Homie compliance phase 3: Controller robustness #93)
• Phase 4: base layer vs higher layer split, and upstream proposals (Homie compliance phase 4: base vs higher layer, upstream proposals #94)

Related: electrification-bus/ebus-mqtt-client#24, exposing the MQTT retain flag to subscribers, needed by Phase 1.

Line numbers refer to homieiot/convention@7edc221.