Transients: mutable backing instead of copy-on-write

The Chez port had landed transients as copy-on-write — each conj!/assoc!/etc.
rebuilt the whole persistent collection. Semantics were right but a transient
vector was O(n^2) to build (the persistent vector is a flat array, so every
conj! copied it); maps/sets were ~O(n log n) since the HAMT only path-copies.
This restores the Janet host's approach: true mutable backing, snapshot once on
persistent!.

  vec : a growable Scheme vector (capacity + fill count); conj!/pop! amortized
        O(1), persistent! hands off the buffer (exact fit) or trims once.
  map : a Chez hashtable keyed by key-hash/jolt= (value equality, nil-safe);
        persistent! folds it into a pmap.
  set : a Chez hashtable; persistent! folds into a pset.
  cow : fallback for anything else (e.g. a sorted coll) keeps the old
        copy-on-write path, preserving jolt's superset.

get/count/contains?/nth see through each representation. Building a 400k vector
went from minutes (quadratic) to ~50ms (linear). assoc! keeps the variadic
dangling-key nil-pad on both vectors and maps. test/chez/transient-test.ss pins
the invariants and the linear-time property; wired in as `make transient`.

Makefile

@@ -4,7 +4,7 @@
 # build step. `make test` is the full gate. `make remint` rebuilds the seed after a
 # source change.
 
-.PHONY: test ci values corpus unit smoke selfhost sci certify ffi remint
+.PHONY: test ci values corpus unit smoke selfhost sci certify ffi transient remint
 
 # Full gate (dev machine). Includes the self-host byte-fixpoint, which only holds
 # on the same Chez that minted the seed.
@@ -15,7 +15,7 @@ test: selfhost ci
 # lockfile) — it RUNS correctly on any Chez, but `selfhost` rebuilds it and a
 # different Chez version may emit byte-different (gensym/order) output, so the
 # byte-fixpoint is a dev-machine check, not a CI one (jolt-8479).
-ci: values corpus unit smoke sci ffi certify
+ci: values corpus unit smoke sci ffi transient certify
 	@echo "OK: CI gates passed"
 
 # Self-host fixpoint: bootstrap.ss rebuild == checked-in seed.
@@ -47,6 +47,10 @@ sci:
 ffi:
 	@chez --script test/chez/ffi-server-test.ss
 
+# Transients: mutable backing, snapshot on persistent!, and linear-time builds.
+transient:
+	@chez --script test/chez/transient-test.ss
+
 # JVM oracle: certify the corpus against reference Clojure. Skips if clojure absent.
 certify:
 	@if command -v clojure >/dev/null 2>&1; then \