docs: capture engine singleton-lock learning (EngineAlreadyRunningError != no engine)

Document the false "engine not running" banner root cause and fix as a
docs/solutions learning, and add an "Engine Singleton Lock" entry to
CONCEPTS.md: a failed per-machine lock acquisition is proof an engine is
running elsewhere, not "no engine."

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CONCEPTS.md

@@ -82,6 +82,9 @@ The core board entity: a unit of work that moves through columns (triage, todo,
 ### Workflow Runtime
 The authoritative task lifecycle runtime. It resolves a Task to workflow IR, walks the graph, routes node outcomes, and invokes runtime primitives for side effects. The engine substrate still owns scheduling, routing claims, persistence, concurrency, process supervision, storage, and audit plumbing; lifecycle policy lives in workflow nodes and built-in workflow IR.
 
+### Engine Singleton Lock
+A per-machine mutual-exclusion guard ensuring only one fusion process runs the engine for a given project, combining a lockfile in the project's `.fusion/` directory with a per-project loopback socket. Failure to acquire it (`EngineAlreadyRunningError`) is **positive proof an engine is already running** for that project elsewhere on the machine — not an error to swallow and not "no engine." A process refused the lock keeps that as a fact: it reports the engine as available (so UI surfaces don't claim it's down) while reconciliation keeps retrying, so it takes over if the current owner exits.
+
 ### ACP Ask Path
 A one-turn read-only model ask routed through the ACP runtime rather than a CLI print mode. The runner accumulates streamed prose, may recover a trailing JSON object for structured seams, and treats abnormal ACP stop reasons as incomplete answers for validator use.