Add shared sequence identity primitives

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Sequence Identity And Combination Implementation Plan
 
-Status: ready for implementation; no stages completed.
+Status: implementation in progress; Stage 1 complete.
 
 Audience: a Luna coding agent implementing one numbered stage per prompt, in
 order. This plan starts at commit
@@ -175,6 +175,9 @@ Validation: `go test ./internal/engine -run 'SequenceIdentity|SequenceNumber'`
 and the everyday check. Retain the old registration path until its adapters are
 replaced; do not generalize unrelated YAML scalar decoding.
 
+**Completion:** Implemented and verified with the focused identity tests and
+`./scripts/check.sh`.
+
 ## Stage 2 — Compile Exact And Dynamic Application Constraints
 
 Scope: private registration model, selector matching, overlap validation.