core: move if-let/if-some/when-some/while/dotimes macros to overlay

Phase 3 batch 2 (jolt-1j0), 5 user-facing macros as defmacro + syntax-quote.
The conditional-binding macros use the auto-gensym temp# idiom so the name binds
only in the taken branch (the else/empty branch sees the outer scope, carrying
forward the earlier scope fix). when-let stays in Janet for now — 20-coll uses
it, so it must remain available before the macro tier loads.

Note: gensym in a macro body resolves to Janet's 0-arity builtin, so explicit
(gensym "prefix") fails — use template auto-gensym (foo#) instead.

conformance 228/228 x3, full suite green (incl. the if-let scope regression spec,
now exercising the overlay macro), clojure-test-suite 3930, bench A/B flat.

jolt-core/clojure/core/30-macros.clj

@@ -16,3 +16,30 @@
 ;; via rest-destructuring.
 (defmacro if-not [test then & [else]]
   `(if (not ~test) ~then ~else))
+
+;; Conditional binding macros: the name is bound ONLY in the taken branch (the
+;; auto-gensym temp# tests the value; the else/empty branch sees the surrounding
+;; scope). temp# is a single template-local gensym — referenced twice, same symbol.
+(defmacro if-let [bindings then & [else]]
+  (let [form (bindings 0) tst (bindings 1)]
+    `(let [temp# ~tst]
+       (if temp# (let [~form temp#] ~then) ~else))))
+
+(defmacro if-some [bindings then & [else]]
+  (let [form (bindings 0) tst (bindings 1)]
+    `(let [temp# ~tst]
+       (if (some? temp#) (let [~form temp#] ~then) ~else))))
+
+(defmacro when-some [bindings & body]
+  (let [form (bindings 0) tst (bindings 1)]
+    `(let [temp# ~tst]
+       (if (some? temp#) (let [~form temp#] ~@body) nil))))
+
+(defmacro while [test & body]
+  `(loop [] (when ~test ~@body (recur))))
+
+(defmacro dotimes [bindings & body]
+  (let [i (bindings 0) n (bindings 1)]
+    `(let [n# ~n]
+       (loop [~i 0]
+         (when (< ~i n#) ~@body (recur (inc ~i)))))))