``_target_to_response`` in ``api/routes/output_targets.py`` used to be
an isinstance ladder over the three OutputTarget subclasses with a
silent fallback that fabricated a ``LedOutputTargetResponse`` for
unknown types (audit finding H3). The fallback masked exactly the
kind of bug we hit on the CSS side in Phase 1.1: a new target subclass
slipped past the ladder and got mis-shaped on the wire.
Replace the ladder with a ``_TARGET_RESPONSE_BUILDERS`` dict keyed by
the concrete subclass plus an import-time
``_assert_target_response_coverage()`` that requires the registry to
exactly match ``{WledOutputTarget, HALightOutputTarget,
Z2MLightOutputTarget}``. ``_target_to_response`` now raises
``RuntimeError`` instead of silently fabricating a LED response for an
unknown subclass — coverage is asserted at import so this branch is
unreachable in normal operation.
Tests: 5 new regression tests cover bijection between expected classes
and registered builders, callable shape, the rogue-target-raises
contract, and missing/extra entry rejection in the assertion. 24
existing output-target tests stay green; ruff clean.
