Derive extends' merge policy from KeySpec#47
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…rge change The Task 2 implementer could not find this file (it existed only as an uncommitted file in the main checkout, which the worktree's fresh base ref did not carry over) and reconstructed a shorter version from context, losing the Motivation, Non-goals, and Risk sections. Restore the original, already-reviewed design content; the finalized summary line is unchanged.
… design doc Final review caught two drift spots in the design file: illustrative code snippets still showed the 2-arg merge(base, local) signature superseded during implementation by the 4-arg (name, key, base, local) form, and the structural-key count said 8 instead of 9.
This was referenced Jul 13, 2026
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Summary
KeySpec(keys.py) gains amergefield, baked in automatically by the_list()/_map()factories, so a future_list/_mapkey can't silently skip extends support.extends.pynow derives concat/merge semantics for the 8SERVICE_KEYS-shaped keys fromspec.mergeinstead of a hand-copied_MERGE_KEYS/_CONCAT_KEYSset, closing a silent-drift failure mode (a forgottenextends.pyentry used to degrade "concatenate" to "last writer wins" with no error).labels/annotations(e.g.labels: ["team=core"]) now merge correctly acrossextends— previously an untested gap that raisedUnsupportedComposeError.KeySpec:environment,extra_hosts,healthcheck,depends_on,secrets,configs,volumes,tmpfs,env_file) keep their existing hand-maintained sets, shrunk and commented, deferring unification perdecisions/2026-07-12-reject-structural-key-registry.md.Full rationale:
planning/changes/2026-07-13.05-derive-extends-merge-policy-from-keyspec.md.Test plan
just test-ci— 401 passed, 100% line coveragejust lint-ci— clean (ruff format/check, ty, eof-fixer, planning check)