docs: defer native multi-session coverage

docs/implementation-plan.v1.md

@@ -750,9 +750,10 @@ Provision at least one primary interactive host with:
 Maintain snapshot/policy variants for logged-out, standard-user active,
 split-token-admin active, UAC disabled/already-full admin, and Administrator
 Protection when supported. The harness must restore the baseline after any
-variant that changes machine policy. A Windows Server Desktop Experience VM with
-multiple simultaneous `WTSActive` sessions is required for the final native
-ambiguous-session case; until available, keep its exhaustive selector unit test
+variant that changes machine policy. Native multi-session/ambiguous-session
+validation requires a later Windows Server Desktop Experience or RDS fixture
+with multiple simultaneous `WTSActive` sessions. It is explicitly deferred for
+the current Windows-first v1 milestone; keep its exhaustive selector unit test
 mandatory and mark only that native case blocked. Add a Server Core VM for the
 headless Session 0 release smoke. Before publishing, also exercise the oldest
 supported Windows 10 or Server 2016 baseline; it need not be the everyday runner.
@@ -903,8 +904,8 @@ Guest Additions and Guest Control have been verified.
 
 This fixture is now available for native testing, so remove any generic
 “Windows host unavailable” skip only when the harness can acquire this VM's
-exclusive lease and perform the reset/health checks above. Keep the missing
-standard-user, ambiguous-session, Server Core, and oldest-supported-build cases
+exclusive lease and perform the reset/health checks above. Keep the deferred
+native multi-session/ambiguous-session, Server Core, and older-build cases
 explicitly represented as separate blocked matrix entries until their own
 resettable fixtures exist.