feat: build-time mutable/immutable collection toggle + fix list mutation

Add src/jolt/config.janet exposing a build-time `mutable?` constant read from
JOLT_MUTABLE at compile time (jpm build). Default is immutable.

Fix correctness bug: conj on a list (Janet array) mutated the original in
place. In immutable mode conj now copies first; in mutable mode it mutates.

Round 1 of persistent-collections work.

src/jolt/config.janet

@@ -0,0 +1,15 @@
+# Build-time collection mode.
+#
+# Jolt can be built with either immutable (persistent) collections — proper
+# Clojure value semantics — or fast Janet-native mutable collections.
+#
+#   jpm build                 # immutable (default)
+#   JOLT_MUTABLE=1 jpm build  # mutable
+#
+# This reads the environment at module-load time, so for a jpm-compiled
+# executable the value is fixed when the binary is built (a true compile flag).
+# `mutable?` is a constant, so the type-mode branches throughout core fold away.
+(def mutable? (= "1" (os/getenv "JOLT_MUTABLE")))
+
+# Convenience: immutable? is the default.
+(def immutable? (not mutable?))

src/jolt/core.janet

@@ -4,6 +4,7 @@
 (use ./types)
 (use ./phm)
 (use ./regex)
+(use ./config)
 
 # ============================================================
 # Predicates
@@ -195,7 +196,8 @@
     (tuple/slice (tuple ;(array/concat (array/slice coll) xs)))
     (if (array? coll)
       (do
-        (var result coll)
+        # lists prepend; copy first unless built in mutable mode
+        (var result (if mutable? coll (array/slice coll)))
         (var i 0)
         (while (< i (length xs))
           (set result (array/insert result 0 (xs i)))