electric: fixes e/amb by propagating siting

This worked

    `(do (e/server (d/pull db x)))`

This didn't

    `(do (e/server (d/pull db x)) nil)`

`(do a b)` expands to `(e/amb (e/drain a) b)`. `e/amb` wraps a and b
into ctors. Ctors clear siting. This fix propagates the site into the
ctors.

src/hyperfiddle/electric_de.cljc

@@ -22,6 +22,7 @@
     (throw (ex-info (str "Electric code (" fn ") inside a Clojure function") (into {:electric-fn fn} (meta &form))))))
 
 (defmacro ctor [expr] `(::lang/ctor ~expr))
+(defmacro sited-ctor [expr] `(::lang/ctor (::lang/site ~(::lang/current &env) ~expr)))
 (defmacro $ [F & args] `(check-electric $ (lang/$ ~F ~@args)))
 
 (defmacro frame []
@@ -107,7 +108,7 @@ Syntax :
 (amb table1 table2 ,,, tableN)
 ```
 Returns the concatenation of `table1 table2 ,,, tableN`.
-" [& exprs] `(::lang/call (join (i/fixed ~@(map #(list `r/invariant (list `ctor %)) exprs)))))
+" [& exprs] `(::lang/call (join (i/fixed ~@(map #(list `r/invariant (list `sited-ctor %)) exprs)))))
 
 (defmacro input "
 Syntax :