Repair longstanding, subtle bug in both old- and new-syndicate.

during/spawn used not to add linkage assertions to its
initial-assertion set. In addition, if a spawned actor died in its
initial boot procedure, its initial assertions would never be visible.
These two problems interlocked to cause a space leak in during/spawn,
where monitoring facets would never be cleaned up.

This change does two things:
 - adds linkage assertions to the initial-assertion set in during/spawn
 - properly briefly signals initial-assertions even when a new actor
   immediately crashes.

Together, these repair the space leak in during/spawn with a crashy
child startup procedure.

syndicate/syntax.rkt

@@ -434,7 +434,9 @@
                (w.wrapper #:linkage [(assert inst)
                                      (stop-when (retracted (observe inst)))]
                           #:name name.N
-                          #:assertions [assertions.exprs ...]
+                          #:assertions [inst
+                                        (observe (observe inst))
+                                        assertions.exprs ...]
                           O ...)))))]))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

syndicate/test/core/death-during-startup.rkt

@@ -0,0 +1,31 @@
+#lang imperative-syndicate/test-implementation
+;; An error signalled during setup of a new actor's root facet must
+;; cause previous actions to be discarded, but must also cause any
+;; initial-assertions, including specifically linkage assertions from
+;; during/spawn, to be briefly visible.
+
+(test-case
+    [(assertion-struct request (id))
+     (assertion-struct response (value))
+
+     (spawn (during/spawn (request $id)
+              (printf "starting request handler\n")
+              (assert (response 'the-answer)) ;; must not be visible
+              (printf "asserted response, shouldn't be visible\n")
+              (error 'aieee "oh no")
+              (printf "NOTREACHED\n")))
+
+     (spawn (stop-when (asserted (observe (request _)))
+              (printf "service listening\n")
+              (react
+               (assert (request 101))
+               (stop-when (retracted (observe (request _)))
+                 (printf "whole service vanished\n"))
+               (stop-when (retracted (observe (request 101)))
+                 (printf "specific instance vanished\n"))
+               (stop-when (asserted (response $v))
+                 (printf "response ~v\n" v)))))]
+  (expected-output (list "service listening"
+                         "starting request handler"
+                         "asserted response, shouldn't be visible"
+                         "specific instance vanished")))