AOT build: set per-ns ns context and register aliases

The source loader sets the current ns and registers :as aliases per file. The
build flattened every app namespace into one image with no such markers, so all
app forms ran under the last-set ns ("user"). Two breakages followed, both only
in a built binary:

- defmulti/defmethod resolve their target var through chez-current-ns, so they
  registered the multifn under "user" while compiled var-derefs used the baked
  ns — the multifn the app saw was uninitialized ("not a fn nil" on dispatch).
- a quoted alias-qualified symbol (a (defmethod ig/foo …) on an aliased multifn)
  resolves its ns through chez-resolve-alias, but the stripped (ns …) form left
  the alias table empty, so it landed in ns "ig".

bld-ns-prelude now emits (set-chez-ns! ns) plus chez-register-alias! for each
ns's :as aliases before that ns's forms, in both the normal and tree-shake emit
paths. The build-app fixture gains a :default multimethod and an aliased cross-ns
defmethod so buildsmoke covers both across all build modes.

test/chez/build-app/src/app/util.clj

@@ -6,3 +6,10 @@
 
 (defmacro twice [x]
   `(do ~x ~x))
+
+;; A multimethod with a :default method. The AOT build must set the per-ns
+;; current ns before these forms run, or the defmethod registers app.util/greet
+;; under the wrong ns and a dispatch to :default crashes (not a fn nil). app.core
+;; adds an aliased method (util/greet :loud) — see there.
+(defmulti greet (fn [kind] kind))
+(defmethod greet :default [_] "greet:default")