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/core.clj

@@ -2,10 +2,17 @@
   (:require [app.util :as util]
             [clojure.java.io :as io]))
 
+;; An aliased cross-ns defmethod: 'util/greet is passed quoted to defmethod-setup,
+;; so the AOT build must register the `util` alias for app.core or it resolves to
+;; ns "util" and never reaches app.util/greet (the dispatch falls to :default).
+(defmethod util/greet :loud [_] "greet:loud")
+
 (defn -main [& args]
   ;; the resource is baked into the binary (deps.edn :jolt/build :embed), so this
   ;; resolves with no resources/ dir on disk, run from any cwd.
   (println (slurp (io/resource "greeting.txt")))
   (util/twice (println (util/shout "hello from a built binary")))
   (println "args:" (vec args))
-  (println "sum:" (reduce + (map count args))))
+  (println "sum:" (reduce + (map count args)))
+  (println "greet-default:" (util/greet :unknown))
+  (println "greet-loud:" (util/greet :loud)))

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")