refactor: rename dynamic-vars.ss, extract natives-format.ss (jolt-7dkx)

Two small clarity moves from the review:
- dynamic-vars.ss -> dynamic-var-defaults.ss. It holds the default VALUES of a few
  core dynamic vars (*clojure-version*, *assert*, …); the near-identical dyn-binding.ss
  holds the binding-stack machinery. The names were easy to confuse.
- Pull the ~60-line %-format engine out of the natives-misc.ss grab-bag into
  natives-format.ss (its ->long/pad-left/fmt-float helpers were local to it).
rt.ss loads + MODULES.md updated. Runtime .ss, no re-mint; make test green, format +
the dynamic vars verified.

host/chez/rt.ss

@@ -272,7 +272,7 @@
 
 ;; dynamic vars: *clojure-version* / *unchecked-math* constants the host
 ;; binds natively. After collections.ss (jolt-hash-map) + def-var!.
-(load "host/chez/dynamic-vars.ss")
+(load "host/chez/dynamic-var-defaults.ss")
 
 ;; host tables + sorted collections: jolt.host/tagged-table/
 ;; ref-put!/ref-get + the 25-sorted tier's runtime (sorted-map/sorted-set routed
@@ -302,6 +302,10 @@
 ;; in post-prelude.ss.
 (load "host/chez/natives-misc.ss")
 
+;; format / printf: the %-directive engine. After natives-misc.ss + converters.ss
+;; (jolt-str-render-one).
+(load "host/chez/natives-format.ss")
+
 ;; namespaces: the namespace value model — find-ns/ns-name/
 ;; all-ns/the-ns/create-ns/in-ns/ns-publics/ns-map/ns-interns/ns-aliases/resolve/
 ;; find-var/ns-unmap/*ns*, over the var-table + chez-current-ns. Loaded LAST: needs