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/index.ts

@@ -203,6 +203,8 @@ export function startInProcessRun(
   try {
     unlink = request.parent.ctx.effect(() => () => handle.dispose())
   } catch (error: unknown) {
+    // Fire-and-forget: start() must rethrow synchronously; the child's
+    // teardown (stop → unregister → detach) reaches quiescence on its own.
     void handle.dispose()
     throw error
   }