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.

docs/MODULES.md

@@ -81,6 +81,10 @@ sync when changing it.
   (stash) + `host-contract.ss` (`inline-ir`/`stash-inline!`); `test/chez/inline-test.ss`.
 - **Multimethods**: `host/chez/multimethods.ss` (dispatch) + the overlay
   `defmulti`/`defmethod` macros + `host-contract.ss` late-bind.
+- **AOT namespace context** (`jolt build`): `build.ss` (`bld-ns-prelude`) emits
+  `(set-chez-ns! ns)` + `chez-register-alias!` per app namespace (both the normal
+  and tree-shake emit paths), matching the loader's per-file ns context;
+  `test/chez/build-app` (`make buildsmoke`).
 - **Deps resolution**: `jolt-core/jolt/deps.clj` (the only file) + `main.clj`
   (applies the roots) + `loader.ss` (the `require` path).