Document and benchmark sibling inheritance

docs/roadmap/deferred.md

@@ -15,7 +15,8 @@ The following remain deliberately excluded:
 
 - automatic `conf.d` discovery;
 - remote imports, arbitrary providers, and additional configuration formats;
-- custom merge strategies, sequence merging, and reset operators;
+- custom merge strategies, sequence merging, reset operators, multiple-parent
+  inheritance, and cross-scope references;
 - named overlay inheritance;
 - parser-specific or custom structural-decoding extension APIs;
 - generic dynamic-subtree conversion and module-registry APIs;
@@ -28,7 +29,6 @@ Applications may build their own validate, effective-value, source, bootstrap, a
 explanation commands around the library. Broadening these boundaries requires
 concrete consumer evidence, compatibility review, and an explicit design change.
 
-Built-in sibling inheritance using `extends` is proposed in the
-[named-inheritance roadmap](named-inheritance.md), based on the
-[downstream request](feature-request-promptkit-inheritance.md). It is not yet
-implemented and is distinct from inheritance between selected overlay files.
+Built-in sibling inheritance using `extends` is implemented as described in the
+[configuration contract](../config.md#sibling-inheritance). It is distinct from
+inheritance between selected overlay files, which remains excluded above.

docs/roadmap/implementation.md

@@ -314,6 +314,15 @@ not a full duplication of every mechanism test. Run `./scripts/check.sh` and
 
 ## Stage 8 — Validate scaling and complete documentation
 
+**Completed 2026-09-29.** Benchmarks cover chains, shared-parent width, nested
+destinations, replacements, and full inspected loads. Skipping repeated scope
+assembly when no immediate relationship exists reduced measured work while a
+single boundary clone preserves caller-input ownership. The maintained preset
+example, configuration contract, consumer guidance, internal navigation,
+architecture/deferred scope, and ADR 0009 now describe the implemented feature.
+`./scripts/check.sh`, `go test -race ./...`, and the focused benchmarks passed.
+No release was tagged or published.
+
 Add focused benchmarks beside assembly/underlay benchmarks: long chains, wide
 shared-parent scopes, nested destination expansion, and replacements. Evaluate
 allocations/work against input, materialized output, and retained history,

docs/roadmap/named-inheritance.md

@@ -1,7 +1,8 @@
 # Built-In Sibling Configuration Inheritance
 
-Status: agreed feature design; not implemented. This document defines the intended
-end state. The [implementation plan](implementation.md) defines execution steps.
+Status: implemented. The [configuration contract](../config.md#sibling-inheritance)
+owns current behavior. This document preserves the agreed design and its
+baseline; the [implementation plan](implementation.md) records completed work.
 
 ## Purpose and priorities
 
@@ -23,7 +24,7 @@ reduced consumer configuration infrastructure. Computational efficiency matters
 within those goals: resolve shared ancestors once and account for the size of
 materialized results and retained history. Avoid an unconditional O(n) promise.
 
-## Current baseline
+## Baseline at design time
 
 Baseline: `b604aa167b684525f0d5d806d9c4267824151ee4`, after the filesystem-source
 and underlay implementation and its remediation.