Chez Phase 2 (inc K): namespace value model (jolt-yxqm)

Reimplement the ctx-coupled seed ns natives over the rt.ss var-table, since
Chez has no ctx. host/chez/ns.ss adds a jns namespace value + a registry and
binds find-ns/the-ns/create-ns/in-ns/all-ns/ns-publics/ns-map/ns-interns/
ns-aliases/resolve/find-var/ns-unmap/*ns* into clojure.core.

The resolve friction: native-ops (+, map, …) are inlined at emit so they have
no var-cell, and (resolve '+) was nil — diverging from Clojure where it's a
var. Added a defined? flag to the var-cell record (set by def-var!/declare-var!,
left false on a lazily-materialised forward ref) and def-var!'d every native-op
name to its value-position proc, so resolve returns the cell iff genuinely
defined. ns-unmap clears the flag. resolve never interns an empty cell
(var-cell-lookup is non-creating).

ns-name is overridden natively in post-prelude (the overlay reads
(get ns :name), nil on a jns record); the printers render a namespace as its
name. *ns* binds to the user ns; in-ns re-binds it. use/require cross-ns
switching stays deferred to Phase 3 (the analyzer bakes a def's target ns at
compile time).

Prelude parity 1951 -> 1969, 0 new divergences; four now-passing allowlist
entries dropped (ns *ns* cases + str-of-a-var). New focused gate test/chez/
_ns.janet (19 cases, expectations from the JVM-canonical build/jolt).

host/chez/jolt-chez.janet

@@ -19,7 +19,8 @@
     (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"
            "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/atoms.ss" "host/chez/predicates.ss" "host/chez/regex.ss"
+           "host/chez/ns.ss" "host/chez/post-prelude.ss"]
     (array/push parts (slurp f)))
   (string/slice (string (hash (string/join parts))) 0))