release: 0.21.0 (declarative device trees + configurable Homie domain)

[dcj]

Bumps __version__ to 0.21.0 (single source of truth), promotes [Unreleased] to [0.21.0], and adds the tag link definition. No code changes.

What's in the release

Issue	Change
#58	PropertySpec property-level parity: round_to, initial_value, retained, internal_only, conditionally_settable, and the source_id / model_group identity splits
#57	DeviceSpec + DeviceTreeBuilder: tree-aware incremental builder over an externally-owned model, late-bound device ids, depth-first teardown
#61	Device(homie_domain=...): publish a tree under any Homie 5 domain

Minor rather than patch: three additive features. Not major: nothing breaks.

The one behavior change is in #58. bind_property_to_homie now binds a non-retained property on-set rather than on-change. It affects only event properties, which previously could not emit a repeated event at all, because the observable model's change gate sat above the Homie layer and swallowed the repeat before the publish-on-change exemption could apply. Retained properties are untouched, and a twin that does not answer retained() is treated as retained.

Verification

• 653 tests pass (600 at 0.20.1, 53 added across the three features)
• ruff check and ruff format --check clean; markdownlint clean on all 7 tracked .md files
• python -m build produces ebus_sdk-0.21.0.tar.gz and ebus_sdk-0.21.0-py3-none-any.whl; wheel METADATA reads Version: 0.21.0 and carries all 14 modules
• Version agrees across all three readers: ebus_sdk.__version__, pyproject.toml's dynamic attr, and the setup.py regex shim

After merge, tagging v0.21.0 triggers publish.yml, which re-checks that the tag equals v + __version__ before publishing anything.

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