core: start Phase 4 — move vary-meta and namespace-munge to the overlay

First Phase 4 batch: the host-coupled fns whose logic is pure composition of
existing core primitives, so no new jolt.host surface is needed. vary-meta is
just (with-meta obj (apply f (meta obj) args)); namespace-munge is a char map
over str. Both land in the 20-coll tier; the Janet core-* defns and bindings
are gone. Added namespace-munge spec cases and a vary-meta extra-args case.

The heavily host-coupled fns (atom/var/transient internals, arrays, proxy,
futures) stay in Janet pending a thin host-primitive expansion.

jolt-core/clojure/core/20-coll.clj

@@ -213,3 +213,15 @@
         (recur (assoc m (first xs) (second xs)) (next (next xs)))
         m))
     s))
+
+;; Phase 4 (jolt-1j0): host-coupled fns that are pure logic over existing core
+;; primitives, so they need no new jolt.host surface.
+
+;; vary-meta: f applied to obj's metadata (+ extra args), reattached. meta and
+;; with-meta are the irreducible host primitives; vary-meta is just their compose.
+(defn vary-meta [obj f & args]
+  (with-meta obj (apply f (meta obj) args)))
+
+;; namespace-munge: Clojure namespace name -> legal Java package name (- -> _).
+(defn namespace-munge [s]
+  (apply str (map (fn [c] (if (= c \-) \_ c)) (seq (str s)))))

src/jolt/core.janet

@@ -1805,8 +1805,6 @@
 (defn core-clojure-version [] "1.11.0-jolt")
 (defn core-munge [s]
   (string/replace-all "-" "_" (string s)))
-(defn core-namespace-munge [s]
-  (string/replace-all "-" "_" (string s)))
 (defn core-test [v]
   (let [t (and (core-meta v) (get (core-meta v) :test))]
     (if t (do (t) :ok) :no-test)))
@@ -2375,8 +2373,8 @@
 # keys must be numbers (NaN allowed) — like Clojure, which compares them with </>.
 # min-key / max-key now live in the Clojure collection tier (core/20-coll.clj).
 
-(defn core-vary-meta [obj f & args]
-  (let [m (core-meta obj)] (core-with-meta obj (apply f m args))))
+# vary-meta / namespace-munge now live in the Clojure collection tier
+# (core/20-coll.clj) — pure compositions of meta/with-meta and str/map.
 
 # Exceptions (ex-info / ex-data / ex-message)
 (defn core-ex-info [msg data & more]
@@ -2876,7 +2874,6 @@
     "associative?" core-associative?
     "ifn?" core-ifn?
     "indexed?" core-indexed?
-    "vary-meta" core-vary-meta
     "ex-info" core-ex-info
     "ex-data" core-ex-data
     "ex-message" core-ex-message
@@ -3009,7 +3006,6 @@
     "class" core-class
     "clojure-version" core-clojure-version
     "munge" core-munge
-    "namespace-munge" core-namespace-munge
     "test" core-test
     "enumeration-seq" core-enumeration-seq
     "iterator-seq" core-iterator-seq

test/spec/metadata-spec.janet

@@ -7,6 +7,7 @@
   ["with-meta preserves value" "true" "(= [1 2 3] (with-meta [1 2 3] {:a 1}))"]
   ["with-meta on map"     "{:doc \"x\"}" "(meta (with-meta {:k 1} {:doc \"x\"}))"]
   ["vary-meta"            "{:a 2}"     "(meta (vary-meta (with-meta [1] {:a 1}) update :a inc))"]
+  ["vary-meta extra args" "{:a 1 :b 2}" "(meta (vary-meta (with-meta [1] {:a 1}) assoc :b 2))"]
   ["meta reader ^"        "{:tag :int}" "(meta ^{:tag :int} [1 2])"]
   ["with-meta on fn ok"   "true"       "(fn? (with-meta inc {:a 1}))"]
   ["with-meta nil clears" "nil"        "(meta (with-meta [1 2 3] nil))"])

test/spec/strings-spec.janet

@@ -48,3 +48,8 @@
   ["subs end past len"  :throws    "(subs \"abcde\" 1 6)"]
   ["subs nil start"     :throws    "(subs \"abcde\" nil 2)"]
   ["subs on nil"        :throws    "(subs nil 1 2)"])
+
+(defspec "string / namespace-munge"
+  ["hyphens to underscores" "\"a_b_c\"" "(namespace-munge \"a-b-c\")"]
+  ["from a symbol"          "\"foo_bar\"" "(namespace-munge (quote foo-bar))"]
+  ["no hyphens unchanged"   "\"ok\""    "(namespace-munge \"ok\")"])