Add roadmap and implementation plan for backend concurrency scaling

docs/roadmap/future.md

@@ -33,41 +33,9 @@ consumers.
 
 ## Ideas
 
-### Backend-specific concurrency management
-
-Extend the
-[LLM backend registry](../consumers/pkg-promptkit.md#register-a-custom-backend)
-with optional per-backend concurrency limits and bounded, buffered admission
-queues. Promptkit could then route simultaneous
-generation requests according to backend capacity while containing accidental
-runaway submission. Downstream consumers would continue invoking synchronous
-`Run` calls, including concurrently from multiple goroutines, and each
-admitted call would wait for and return its ordinary result.
-
-- Scope limits to an engine instance rather than hidden process-global state.
-- Give different backend IDs independent capacity pools. A profile endpoint
-  override would remain part of its selected backend's pool.
-- Configure active concurrency and waiting capacity separately. Concurrency
-  protects the backend, while queue capacity protects the process from
-  admitting an unbounded backlog.
-- Give queue capacity a generous, configurable bounded default intended as a
-  safety ceiling for bugs or unintended loops rather than a routine
-  application constraint. Select an exact default during implementation
-  planning and measurement.
-- Reject a call with a recognizable capacity error when its backend queue is
-  full rather than allowing it to wait outside the bounded queue.
-- Admit requests before expensive preparation and artifact copying where
-  practical so queued work remains lightweight.
-- Apply a limit to each actual generation request, including repair attempts,
-  without unnecessarily serializing prompt preparation.
-- Make queued and active waits respect caller cancellation and deadlines.
-- Treat concurrency as backend policy rather than a profile-level model
-  setting.
-- Keep the queue ephemeral and in-process, with no survival guarantee across
-  engine or process shutdown.
-- Preserve the existing execution model as far as practical. Durable jobs,
-  polling, priorities, application worker lifecycle, retries, and
-  cross-process coordination would be separate future capabilities.
+No ideas are currently cataloged. Backend-specific concurrency management has
+been selected for active planning in the
+[focused concurrency roadmap](concurrency.md).
 
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