Add configuration comparison capability contract

docs/roadmap/implementation.md

@@ -124,10 +124,20 @@ ordinary acquisition for a comparison-only scope.
    when applicable to a comparison-capable scope. Rely on existing core binding
    validation for collisions with generated flags.
 
-**Completion evidence:** focused installation/discovery checks for comparison-only,
-mixed, unsupported, and metadata-only scopes; missing callback and retained nil
-loader issues; captured metadata surviving source edits. Execution is wired in
-Stages 3, 4, and 8; do not add temporary handlers for intermediate completeness.
+**Completion evidence — complete:** `Diff`, `ComparisonSide[I]`,
+`ComparisonRequest[I]`, and the optional `Scope.Compare` callback are declared in
+ckconfig. Capability validation now requires `Acquire` only for ordinary
+operations and `Compare` only for `Diff`; retained adapter issues still reach
+installation after capability narrowing. Operation normalization and discovery
+preserve the four existing operation order and append `diff`, generating the
+family command only when a scope declares comparison. Focused checks cover
+comparison-only, mixed, unsupported, and metadata-only scopes; missing provider
+wiring; a narrowed ConfigKit adapter retaining its nil-loader issue; and captured
+capability metadata and provider references surviving edits to the source
+declaration. Construction, installation, discovery, and unsupported-scope
+preflight leave factories and providers uninvoked. The ckconfig and dependent
+ckp suites pass. Input flags and successful comparison execution remain for
+their later owners.
 
 ## Stage 3: Add comparison flags, inheritance, and availability preflight