test: close the per-file coverage gaps for the scoping surface

Every subject-extractor row of the invariants carrier table is exercised
with a matching and a foreign-keyed carrier; the HMR re-apply seed path
(sessions of agents that predate the plugin are marked started) is pinned;
the scoped tool-provider disposal, plural restrict() validation, singular
scopeHost absentee, tool-subagent passthrough, stale-stage drop, and
disposing-parent spawn (INACTIVE_EFFECT, no orphan) each gain their test.
Two genuinely defensive branches carry justified v8-ignore markers.

packages/subagent/subagent-inprocess/src/structured.ts

@@ -184,6 +184,9 @@ export function attachStructuredRuntime(childCtx: Context, schema: StructuredOut
       if (decision.kind === 'accept') captured = { value: staged.value }
       return decision
     } finally {
+      /* v8 ignore next -- defensive false branch: a concurrent re-stage
+       * would need a second capture call INSIDE the first's post-execute
+       * chain */
       if (pending === staged) pending = undefined
     }
   }, { prepend: true })