Chez Phase 3 inc 4: swap jolt-chez to the jolt.backend-scheme emitter; full corpus parity

driver.janet now compiles IR via the portable Clojure emitter (jolt.backend-
scheme) instead of emit.janet, at every entry point (compile-program, emit-core-
prelude, eval-e-with-prelude). The emitter is loaded into the ctx and called like
the analyzer. emit.janet stays only as the emit/program string-wrapper until
program assembly ports to Clojure with compile-from-source; its emit fn is no
longer called anywhere (emit-test's truthy-elision helper now uses the new
d/scheme-emit too). This takes the IR->Scheme emitter off Janet.

A form-by-form diff of the two emitters over the whole prelude found one gap:
emit-const missed char literals because a :jolt/type-tagged struct is not a plain
jolt map? — switched to the form-char? host contract. Diff then 0.

jolt-chez prelude fingerprint now includes backend_scheme.clj + host_iface.janet.

Gate: full prelude corpus 2280/2494, NEW divergence 0, same buckets as the Phase-2
emit.janet floor (36 emit-fail, 170 crash) — the Clojure emitter is byte-for-
behavior identical. emit-test 331/331 (now via the Clojure emitter), emit-parity
58/58. jolt-duot.

host/chez/jolt-chez.janet

@@ -17,7 +17,8 @@
   (def parts @[])
   (each tf d/core-tier-files
     (array/push parts (slurp (string "jolt-core/clojure/core/" tf ".clj"))))
-  (each f ["host/chez/emit.janet" "host/chez/driver.janet" "host/chez/rt.ss"
+  (each f ["jolt-core/jolt/backend_scheme.clj" "src/jolt/host_iface.janet"
+           "host/chez/emit.janet" "host/chez/driver.janet" "host/chez/rt.ss"
            "host/chez/values.ss" "host/chez/collections.ss" "host/chez/seq.ss"
            "host/chez/atoms.ss" "host/chez/predicates.ss" "host/chez/regex.ss"
            "host/chez/ns.ss" "host/chez/post-prelude.ss" "host/chez/natives-meta.ss"