Make the benchmark harness build optimized binaries on Chez (#220)

bench/run.sh was Janet-era: it invoked a 'jolt' binary and set
JOLT_DIRECT_LINK/JOLT_WHOLE_PROGRAM, none of which exist on Chez, where
'joltc run -m' runs fully unoptimized (direct-link and inline default off). So
the suite was measuring jolt's unoptimized path.

run.sh now compiles each benchmark to an optimized AOT binary (joltc build
--direct-link --opt) and times it against JVM Clojure on the same portable
source, auto-detecting the Chez kernel dev files like build-smoke.sh. Adds
bench/deps.edn so joltc resolves the namespaces, NO_JVM to skip the reference.

mandelbrot.clj dropped its jolt.png require so the JVM reference can run it; the
picture demo moved to mandelbrot_png.clj (jolt-only). README scorecard refreshed
with current Chez numbers and the two-regime read (compute ~8-10x substrate floor;
dispatch/alloc ~120-330x architectural gaps the passes don't touch). Stale
'jolt -m' header lines point at bench/run.sh.

Co-authored-by: Yogthos <yogthos@gmail.com>

bench/mandelbrot_png.clj

@@ -0,0 +1,36 @@
+;; mandelbrot picture demo — renders a real image of the set to a PNG via
+;; jolt.png (FFI), reusing mandelbrot/count-point as the kernel. jolt-only (the
+;; benchmark in mandelbrot.clj stays portable for the JVM reference).
+;;   joltc run -m mandelbrot-png [path] [size]
+(ns mandelbrot-png
+  (:require [mandelbrot :as m]
+            [jolt.png :as png]))
+
+(defn- color
+  "Escape-iteration count -> RGB. In-set points (n>=cap) are black; faster
+  escapes run through a warm gradient."
+  [n cap]
+  (if (>= n cap)
+    [0 0 0]
+    (let [t (/ (double n) cap)]
+      [(int (* 255 (min 1.0 (* 3.0 t))))
+       (int (* 255 (min 1.0 (max 0.0 (* 3.0 (- t 0.33))))))
+       (int (* 255 (min 1.0 (max 0.0 (* 3.0 (- t 0.66))))))])))
+
+(defn render!
+  "Render a size×size view of the Mandelbrot set to a PNG at path."
+  [path size]
+  (let [w size h size cap 1000
+        img (png/image w h)]
+    (doseq [py (range h)]
+      (doseq [px (range w)]
+        (let [cr (- (* 3.5 (/ (double px) w)) 2.5)        ; real ∈ [-2.5, 1.0]
+              ci (- (* 2.8 (/ (double py) h)) 1.4)        ; imag ∈ [-1.4, 1.4]
+              [r g b] (color (m/count-point cr ci cap) cap)]
+          (png/put! img r g b))))
+    (png/write img w h path)
+    (println "wrote" path (str w "×" h ", cap " cap))))
+
+(defn -main [& args]
+  (render! (or (first args) "mandelbrot.png")
+           (if (second args) (Integer/parseInt (second args)) 600)))