docs: clarify stdio process lifetime in lifecycle spec

Add prescriptive guidance that the stdio process should track the host
application lifetime and serve multiple tasks, threads, or conversations,
rather than being scoped to any single one.

Author: kurtisvg

docs/specification/draft/basic/lifecycle.mdx

@@ -20,11 +20,17 @@ Specifically:
   establish context (e.g., capabilities, protocol version, client identity).
   Every request supplies this metadata in its
   [`_meta`](/specification/draft/basic/index#meta) field.
-- Servers **MUST NOT** require that a client reuse the same connection to
-  perform related operations.
+- Servers **SHOULD** be prepared to handle requests associated with multiple
+  tasks, threads, or conversations. Servers **SHOULD NOT** require that a client
+  reuse the same connection or process to perform related operations. Clients
+  **SHOULD NOT** use an individual task, thread, or conversation as the default
+  lifetime boundary for the stdio process.
 - State that needs to span multiple requests (e.g., long-running tasks,
   application-level handles) **MUST** be referenced by an explicit identifier
   the client passes on each request.
+- The stdio process **SHOULD** have approximately the same lifetime as the host
+  application instance that launched it. Clients **SHOULD** attempt to restart
+  the stdio process if the server terminates unexpectedly.
 
 Long-lived requests like
 [`subscriptions/listen`](/specification/draft/basic/utilities/subscriptions)