Document backend capacity management

docs/roadmap/concurrency.md

@@ -1,6 +1,6 @@
 # Backend-Specific Concurrency Management
 
-**Status:** Accepted.
+**Status:** Complete.
 
 ## Purpose
 

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Backend-Specific Concurrency Management Implementation Plan
 
-**Status:** Ready for implementation.
+**Status:** Complete.
 
 ## Purpose
 
@@ -647,7 +647,7 @@ releases admission, and existing preparation semantics remain unchanged.
 
 ## Stage 4 — Engine Assembly And Public Runtime Contract
 
-**Status:** Pending.
+**Status:** Complete.
 
 ### Goal
 
@@ -721,7 +721,7 @@ through the same wrapper.
 
 ## Stage 5 — Durable Documentation And Final Validation
 
-**Status:** Pending.
+**Status:** Complete.
 
 ### Goal
 
@@ -819,6 +819,19 @@ limits and cancellation safety, durable contracts no longer depend on roadmap
 prose, and the OpenRouter compatibility change is clearly reported for the
 next minor release.
 
+## Implementation Handoff
+
+Backend-specific capacity management is implemented and has passed the complete
+repository validation sequence. The built-in OpenRouter backend now permits 16
+active generations and a waiting capacity of 1024. Custom backends remain
+unlimited when their limit is omitted, and endpoint-only profiles remain
+unlimited.
+
+Publishing this behavior requires a pre-`v1` minor release. Its release notes
+must identify that unusually high concurrent OpenRouter use can now wait or
+return `ErrCapacityExceeded`. This implementation does not change a module
+version or create a tag.
+
 ## Open Questions
 
 None. The feature roadmap and this plan fix the public representation,