Chez re-host spike: substrate ceiling vs Janet (fib/mandelbrot)

Hand-translate the two compute benches into the Scheme a jolt->Chez backend
would emit, to localize the execution-substrate ceiling without porting the RT.

fib 30: 246.6 -> 5.2 ms (~47x, fixnum). mandelbrot 200: 166.3 -> 13.4 ms
(~12.4x) ONLY with flonum-specialized ops; generic float ops box every flonum
and stay ~1.7x. 13.4 ms matches jolt's JOLT_CGEN C result, so Chez's native
compiler reaches the C ceiling with no cc step, REPL intact.

Size: Chez base 2.9 MB (AOT) / 4.0 MB (dynamic) vs Janet 2.21. Memory: Chez
~32-49 MB fixed baseline vs Janet ~12 MB (the one regression). RT-bound axes
(collections/binary-trees, where Chez's generational GC should help) not yet
measured. See spike/chez/RESULTS.md.

spike/chez/fib.ss

@@ -0,0 +1,25 @@
+;; fib spike — translated from bench/fib.clj. Pure call + integer arith.
+;;   chez --script fib.ss [n=30] [optlevel=2]
+(import (chezscheme))
+(optimize-level
+  (let ((a (command-line-arguments)))
+    (if (and (pair? a) (pair? (cdr a))) (string->number (cadr a)) 2)))
+
+(define (fib n) (if (< n 2) n (+ (fib (- n 1)) (fib (- n 2)))))
+(define (run n) (fib n))
+
+(define (now-ns)
+  (let ((t (current-time 'time-monotonic)))
+    (+ (* (time-second t) 1000000000) (time-nanosecond t))))
+
+(let* ((a (command-line-arguments))
+       (n (if (pair? a) (string->number (car a)) 30)))
+  (run (- n 6)) (run (- n 6))                              ; warmup
+  (let loop ((k 0) (acc '()))
+    (if (< k 3)
+        (let* ((t0 (now-ns)) (r (run n)) (ms (/ (- (now-ns) t0) 1000000.0)))
+          (loop (+ k 1) (cons ms acc)))
+        (begin
+          (printf "fib n ~a result ~a\n" n (run n))
+          (printf "runs: ~a\n" (reverse acc))
+          (printf "mean: ~a ms\n" (exact->inexact (/ (apply + acc) 3.0)))))))