fix(agent-loop): let a concluding tool result beat same-step steering

concludesTurn is terminal: a completed step now carries the concluded fact
to the driver, which ends the turn immediately instead of letting steering
that arrived during the tool batch (or from an agent/stopping listener)
reopen it. The steering is not lost — it drained into the log as
steering/message before the decision point, so it feeds the next turn's
request. Also annotates the driver's unreachable invariant guards
(exhaustiveness default, abort-slot ownership) for the coverage gate.

docs/architecture.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write
-architecture.md: 40d1641e494d2087e5fb5f9c50dbe1a2df30b103
-architecture.zh.md: 324c4ff1950cbe3a3b8f015bb545dad3869dd5b4
+architecture.md: 514a74534df4761c2677498b435d0f6cd22e5cd8
+architecture.zh.md: c7e88e6f08d361e4cec15b51046145e88ceff5f6

docs/architecture.md

@@ -100,6 +100,7 @@ forever:
         each model-order result -> ordered tools/post-execute -> 'tool/result'
       drain accepted tool context and steering
       'step/end'
+      a concluding tool result ends the turn; drained steering waits for the next turn
       continue when tools or steering require another step
       otherwise agent/stopping -> drain once more -> continue only for steering
     'turn/end' -> agent/idle

docs/architecture.zh.md

@@ -100,6 +100,7 @@ forever:
         each model-order result -> ordered tools/post-execute -> 'tool/result'
       drain accepted tool context and steering
       'step/end'
+      a concluding tool result ends the turn; drained steering waits for the next turn
       continue when tools or steering require another step
       otherwise agent/stopping -> drain once more -> continue only for steering
     'turn/end' -> agent/idle