Fix actor startup and registration identity

[cevr]

Calling actor.start again could rerun recovery and replace a terminal lifecycle with Starting. Closing an earlier actor scope could also remove a replacement actor that reused its ID.

Share one startup result per actor. Check the registered actor identity before scope cleanup removes an ID. A patch changeset records both fixes.

1. Startup ownership:

 actor.start
-  recovery → inspection → supervisor → runtime.start (every call)
+  shared startup result
+    first call: terminal guard → recovery → inspection → supervisor → runtime.start
+    later calls: await or return the stored result

2. Registry cleanup:

 old owner scope closes
-  ID exists → remove ID
+  ID still belongs to old actor → remove ID
   stop old actor

Validation: both regressions failed before the fix. Full gate passes type checks, lint, formatting, build, examples, and 370 tests with 794 assertions. Tests cover concurrent recovery, repeated start, stop before start, final exit, replacement actor identity, replacement during a stop listener, and retained startup interruption. Counsel round one found no blocker. Its cleanup ordering and documentation suggestions are included.