Per RFC-WS-6 §V1 + ARCH-BINDINGS §4.2.
Implements the strategy x target-type validity matrix. Append is the
only non-trivial case: valid only for COLLECTION targets. The
AppendStrategyRequiresCollectionTarget publish-guard uses this method
(D2 wiring confirms call sites; this commit provides the building block).
Existing methods (nullWinnerBehaviour, isValidForScalarTargets) untouched.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>