implement-spf-feature

# Implement an SPF feature Read `references/workflow.md` completely before acting; it contains the detailed disambiguation, planning, implementation, testing, and registry-update workflow. Treat the feature entry as planning input. Reconcile it with code, tests, conventions, and requested scope before editing. 1. Read the entry, related features, implementation, tests, and applicable SPF conventions. 2. List contradictions, open decisions, and out-of-scope capability. 3. Define the smallest end-to-end increment with observable value and split it into testable state, behavior, composition, adapter, and documentation chunks. 4. For each chunk, add or update a failing test, implement the minimum at the correct ownership layer, and run the narrow target. 5. Run cumulative SPF tests, build, and size checks proportional to impact. 6. Collapse the feature record to final decisions, remaining scope, and current source pointers. Report implemented and deferred scope, test evidence, record updates, and remaining risks. Do not claim completion for a prerequisite or partial increment. ## Example Input: “Implement the smallest useful increment of timeline discontinuity support.” Output: The smallest end-to-end capability, test evidence, proportional checks, updated feature status, and explicit deferred scope.