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.

host/chez/build-smoke.sh

@@ -44,7 +44,9 @@ want='embedded resource ok
 HELLO FROM A BUILT BINARY!
 HELLO FROM A BUILT BINARY!
 args: [alpha bb ccc]
-sum: 10'
+sum: 10
+greet-default: greet:default
+greet-loud: greet:loud'
 if [ "$got" != "$want" ]; then
   echo "  FAIL: binary output mismatch"
   echo "--- want ---"; echo "$want"