Notify replaced plugin processes at upgrade grace start

Send rpc.superseded with the newly active hash before activation returns.
Add an opt-in SDK hook and drain helper that closes SDK listeners and idle
connections without cancelling active handlers or background work.
Document the additive notice and preserve existing grace, stop and restart rules.

Cover immediate notices, ignored notices, early service restarts, SDK draining
and duplicate-hook diagnostics.

Card: SAU-352

sdk/tests/ui/superseded.rs

@@ -0,0 +1,11 @@
+struct Supersedes;
+#[sausage_plugin::rpc]
+impl Supersedes {
+    /// First superseded hook.
+    #[sausage_plugin::on_superseded]
+    fn a(&self) -> sausage_plugin::Result<()> { Ok(()) }
+    /// Second superseded hook.
+    #[sausage_plugin::on_superseded]
+    fn b(&self) -> sausage_plugin::Result<()> { Ok(()) }
+}
+fn main() {}