release: 0.20.0 (publish-on-change gate + declare_lost teardown)

[dcj]

Top of the stack. Merge order: #51 → #52 → #53 → this.

Bumps __version__ to 0.20.0 (the single source of truth; pyproject.toml reads it dynamically and the setup.py shim parses the same literal), promotes the Unreleased section to [0.20.0], and adds the compare/tag link definitions — which nothing in CI verifies, so they are the easy ones to forget.

What's in it

Publish-on-change for property values (#50). A retained property whose wire payload is byte-identical to the one it last published is no longer republished. The comparison is on the final payload, after rounding, coercion and empty-string encoding, which is why the SDK owns it: a caller holding a raw reading cannot know whether it will change what goes on the wire. The retained state on the broker is byte-for-byte identical either way; there are simply fewer messages. Non-retained (event) properties are never gated, retraction always publishes, and every whole-tree republish forces, so a broker restarted with an empty retained store is still fully repopulated.

declare_lost() and stop(announce=False) (#46). Device modeled three teardowns and implemented one. A producer that knows it is dying can now announce lost instead of announcing disconnected (a lie) or reaching around the SDK to its client — which is what ebus-panel-sim had to do, and where four downstream bugs came from.

Four stale doc claims corrected (#53), including a README dependency floor that had been wrong since 0.18.0. The local build confirms the fix: Requires-Dist: ebus-mqtt-client>=0.4.0.

Pre-flight

• 596 tests pass (560 at 0.19.0), ruff check + ruff format --check clean on 0.16.1, every Markdown file passes markdownlint.
• python -m build produces ebus_sdk-0.20.0.tar.gz and ebus_sdk-0.20.0-py3-none-any.whl with Version: 0.20.0, so the publish workflow's tag-equals-version check will pass for v0.20.0.
• Both feature branches were mutation-tested: every load-bearing line fails at least one test when broken.

Pushing the v0.20.0 tag after this merges is what triggers the PyPI publish.

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