test(subagent): close continuable coverage and drop unreachable guards

Restores the one-shot settleRun coverage in its own file beside the helper,
covers fork's seed contribution, the post-transfer rollback, the descriptor
model route on cold resume, manager-unload drain, and a failing teardown branch.

Removes three redundant checks the surrounding contracts already own: the
duplicate-Activation and live-id pre-checks (AgentRegistry.enter is the
authoritative collision boundary) and a rollback lifecycle edge that could never
publish because the epoch had no start edge.

docs/core-data-structures/subagent.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 docs/core-data-structures/subagent.md
-subagent.md: 8f24afec47a970711aae49cae6b3535b9f532e5f
-subagent.zh.md: 50c5cb887ef814c074a85fc4fee9cd2fe85d685c
+subagent.md: a58ecf13ba1f5df0e8e35c793eaf9aefc1e8a900
+subagent.zh.md: 541eace7fc6c8ae10ee22639680918e12d7762b3