Add PromptKit configuration comparison

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Implement PromptKit comparison and catalogs
 
-Status: Stages 1–2 complete; remaining stages are planned.
+Status: Stages 1–3 complete; remaining stages are planned.
 
 ## Scope and authority
 
@@ -210,6 +210,28 @@ side diagnostics. Exercise reachable native failures without adding a public
 comparator injection hook solely for tests. Add no reconstructed configuration
 model or duplicate ckprompt comparison renderer.
 
+**Completion evidence (2026-10-12)**
+
+- `ConfigScope` now declares `ckconfig.Diff` and supplies a pair callback that
+  loads left then right with each side's selection/input and the shared state,
+  collects diagnostics in order, checks cancellation at operation boundaries,
+  and returns PromptKit's native `ConfigurationInspection.Diff` records. Ordinary
+  acquisition and the retained nil-loader issue remain available.
+- Added fast native-engine adapter coverage for the PromptKit result, equal
+  selections reaching both loader calls, side inputs/shared state, left failure
+  stopping right, right ConfigKit load diagnostics and error identity, and
+  cancellation stopping before right load. `GOTOOLCHAIN=local go test -count=1 .`
+  passed in `integrations/ckprompt`.
+- PromptKit v0.14.0 exposes no public way to add successful inspection warnings,
+  and valid engine-derived inspection views do not expose a reachable native Diff
+  failure. Therefore successful left-side warning retention on right failure,
+  both-side warning retention on native failure, and a native Diff failure are
+  not exercisable without adding a test-only injection path. The implementation
+  accumulates reached diagnostics and preserves error causes; these limits remain
+  recorded alongside Stage 2's warning-retention limitation.
+- Installed-command verification and the composed consumer's obsolete negative
+  comparison expectation remain for Stages 4 and 9 respectively.
+
 ## Stage 4: Verify comparison through installed config commands
 
 **Outcome:** the adapter works through public assembly, existing typed-input