Chez Phase 2 (inc M): dynamic var binding (jolt-2o7x)

Add the dynamic-binding cluster on the Chez RT: a per-thread binding stack
(host/chez/dyn-binding.ss) backing binding / with-bindings* / var-set /
thread-bound? / with-local-vars / with-redefs / bound-fn* /
get-thread-bindings / alter-var-root + the __local-var seam.

Frames are stored innermost-first as identity-keyed alists of mutable
(cell . value) pairs, so var-set updates the current binding in place. The
two var-read paths — var-deref (compiled code) and jolt-var-get (var-get /
deref on a cell) — are chained onto the stack so a `binding` frame is seen
by every read, with a fast path when the stack is empty. var-cells now hash
by ns/name so a var works as a map key (with-redefs builds (hash-map (var f)
v); get-thread-bindings returns a var-keyed map).

Fix a latent bug exposed once with-in-str could bind *in*: seq? didn't
recognize a lazy-seq. predicates.ss's jolt-seq? predates the lazyseq record,
and unlike the native-op dispatchers it's reached via var-deref, so the
patch must re-def-var! the var, not just set! the top-level binding.

Note: with-bindings* over a hash-map literal now returns the correct Clojure
value where the seed returns a stale one — the seed's PHM can't find a var
key (which is why its `binding` uses array-map); on Chez frames look up by
cell identity.

Prelude corpus parity 1972 -> 2000, floor raised. Gate: _dynbind 24/24,
prelude corpus 0 divergences, full jpm test, conformance 355x3.

host/chez/dyn-binding.ss

@@ -0,0 +1,139 @@
+;; dynamic var binding (jolt-2o7x, Phase 2) — binding / with-bindings* / var-set /
+;; thread-bound? / with-local-vars / with-redefs / bound-fn* / get-thread-bindings.
+;;
+;; A per-thread dynamic-binding stack: a list of frames, innermost (most recently
+;; pushed) at the HEAD. Each frame is an alist of (var-cell . value) MUTABLE pairs
+;; — so var-set can update the innermost binding in place (set-cdr!), matching
+;; Clojure where var-set targets the current binding, not the root.
+;;
+;; The binding macro builds a frame as a jolt map (array-map of (var x) -> value);
+;; push-thread-bindings folds it into the alist. Lookups walk frames by cell
+;; IDENTITY (eq?) — vars are interned, so (var x) always yields the same cell, and
+;; this sidesteps the seed's persistent-hash-map-can't-find-a-var-key quirk.
+;;
+;; var reads (var-deref in compiled code, jolt-var-get / deref on a cell) consult
+;; the stack before falling back to the cell root. Loaded LAST (after vars.ss and
+;; ns.ss) so it chains the fully-extended jolt-var-get and overrides rt.ss var-deref.
+
+(define dyn-binding-stack '())
+
+;; find the innermost (cell . value) pair binding CELL, or #f.
+(define (dyn-find-binding cell)
+  (let loop ((frames dyn-binding-stack))
+    (and (pair? frames)
+         (or (assq cell (car frames))
+             (loop (cdr frames))))))
+
+;; a unique sentinel: distinguishes "no thread binding" from a binding whose
+;; value happens to be jolt-nil.
+(define dyn-no-binding (list 'no-binding))
+(define (dyn-binding-value cell)
+  (if (pair? dyn-binding-stack)
+      (let ((p (dyn-find-binding cell)))
+        (if p
+            (let ((val (cdr p)))
+              (if (var-cell? val) (jolt-var-get val) val))  ; nested var deref (Clojure)
+            dyn-no-binding))
+      dyn-no-binding))
+
+;; push-thread-bindings: frame is a jolt map of var-cell -> value. Fold it into an
+;; identity-keyed alist of mutable pairs and push.
+(define (jolt-push-thread-bindings frame)
+  (set! dyn-binding-stack
+        (cons (pmap-fold frame (lambda (k v acc) (cons (cons k v) acc)) '())
+              dyn-binding-stack))
+  jolt-nil)
+
+(define (jolt-pop-thread-bindings)
+  (when (pair? dyn-binding-stack)
+    (set! dyn-binding-stack (cdr dyn-binding-stack)))
+  jolt-nil)
+
+;; get-thread-bindings: a jolt map of every currently-bound cell -> value,
+;; innermost wins. Merge oldest-frame-first (the stack head is innermost). The
+;; result can be re-pushed by with-bindings* / bound-fn*.
+(define (jolt-get-thread-bindings)
+  (let loop ((frames (reverse dyn-binding-stack)) (m (jolt-hash-map)))
+    (if (null? frames)
+        m
+        (loop (cdr frames)
+              (let frame-loop ((alist (car frames)) (m m))
+                (if (null? alist)
+                    m
+                    (frame-loop (cdr alist)
+                                (pmap-assoc m (caar alist) (cdar alist)))))))))
+
+;; __thread-bound? — single var; true iff it has a thread binding.
+(define (jolt-thread-bound? v)
+  (and (var-cell? v) (dyn-find-binding v) #t))
+
+;; var-set: update the innermost frame that binds v (in place); else set the root.
+(define (jolt-var-set v val)
+  (if (var-cell? v)
+      (let ((p (dyn-find-binding v)))
+        (if p
+            (begin (set-cdr! p val) val)
+            (begin (var-cell-root-set! v val) (var-cell-defined?-set! v #t) val)))
+      (error #f "var-set: not a var" v)))
+
+;; alter-var-root: atomically apply f to the current root plus args.
+(define (jolt-alter-var-root v f . args)
+  (let ((new (apply jolt-invoke f (var-cell-root v) args)))
+    (var-cell-root-set! v new)
+    (var-cell-defined?-set! v #t)
+    new))
+
+;; __local-var: a fresh free-standing var cell (not interned). with-local-vars
+;; binds these as lexical locals; var-get/var-set read/write the root. Each gets a
+;; unique name so two locals never compare/hash equal as map keys.
+(define local-var-counter 0)
+(define (jolt-local-var . args)
+  (set! local-var-counter (fx+ local-var-counter 1))
+  (make-var-cell "" (string-append "local-" (number->string local-var-counter))
+                 (if (pair? args) (car args) jolt-nil)
+                 #t))
+
+;; --- chain the var-read paths onto the binding stack -------------------------
+
+;; var-deref (rt.ss): the compiled-code read path for every clojure.core var
+;; reference. Consult the stack first; fall straight back to the root (NOT through
+;; jolt-var-get's unbound-error path) so undefined-var reads keep prior behaviour.
+(define %dyn-rt-var-deref var-deref)
+(set! var-deref
+  (lambda (ns name)
+    (let ((cell (jolt-var ns name)))
+      (let ((bv (dyn-binding-value cell)))
+        (if (eq? bv dyn-no-binding) (var-cell-root cell) bv)))))
+
+;; jolt-var-get (vars.ss): the var-get fn + deref/@ on a cell. Stack first, then
+;; the original (which errors on an unbound root, matching Clojure).
+(define %dyn-var-get jolt-var-get)
+(set! jolt-var-get
+  (lambda (v)
+    (if (var-cell? v)
+        (let ((bv (dyn-binding-value v)))
+          (if (eq? bv dyn-no-binding) (%dyn-var-get v) bv))
+        (%dyn-var-get v))))
+
+;; var-cell keys hash/compare by ns/name (jolt=2 in vars.ss already compares
+;; ns/name) — stable under root mutation, so a var works as a map key (with-redefs
+;; builds (hash-map (var f) v); get-thread-bindings returns a var-keyed map).
+(define %dyn-hash jolt-hash)
+(set! jolt-hash
+  (lambda (x)
+    (if (var-cell? x)
+        (equal-hash (cons (var-cell-ns x) (var-cell-name x)))
+        (%dyn-hash x))))
+
+;; --- bind the host seams the overlay references -----------------------------
+(def-var! "clojure.core" "push-thread-bindings" jolt-push-thread-bindings)
+(def-var! "clojure.core" "pop-thread-bindings" jolt-pop-thread-bindings)
+(def-var! "clojure.core" "get-thread-bindings" jolt-get-thread-bindings)
+(def-var! "clojure.core" "__thread-bound?" jolt-thread-bound?)
+(def-var! "clojure.core" "var-set" jolt-var-set)
+(def-var! "clojure.core" "alter-var-root" jolt-alter-var-root)
+(def-var! "clojure.core" "__local-var" jolt-local-var)
+;; re-assert var-get / deref to the new (stack-aware) closures (vars.ss captured
+;; the pre-chain values).
+(def-var! "clojure.core" "var-get" jolt-var-get)
+(def-var! "clojure.core" "deref" jolt-deref)