spec: format — CL-style string formatting (~a ~s ~d ~x ~o ~b ~f ~% ~& ~~ ~t)

28 tests, passes on both JS and OCaml.
- spec/stdlib.sx: pure SX format function
- spec/primitives.sx: format primitive declaration
- lib/r7rs.sx: fix number->string to support optional radix arg
- hosts/ocaml: add format-decimal primitive, load stdlib.sx in test runner
- hosts/javascript: load stdlib.sx in test runner

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

spec/stdlib.sx

@@ -0,0 +1,134 @@
+;; ==========================================================================
+;; stdlib.sx — Pure SX standard library functions
+;;
+;; Loaded by test runners after primitives. These functions are implemented
+;; in SX and require no host-specific code.
+;;
+;; IMPORTANT: SX let/when bodies evaluate only the LAST expression.
+;;   Multi-step bodies must be wrapped in (do expr1 expr2 ...).
+;; ==========================================================================
+
+;; --------------------------------------------------------------------------
+;; format — CL-style string formatting
+;;
+;; Directives:
+;;   ~a  display (no quotes)       ~s  write (with quotes)
+;;   ~d  decimal  ~x  hex  ~o  octal  ~b  binary
+;;   ~f  fixed-point (6dp)          ~%  newline
+;;   ~&  fresh line                 ~~  literal tilde
+;;   ~t  tab
+;;
+;; Signature: (format template arg...) -> string
+;; --------------------------------------------------------------------------
+
+(define
+  (format template &rest args)
+  (let
+    ((buf (make-string-buffer)) (n (string-length template)))
+    (define
+      (consume-arg args)
+      (if
+        (nil? args)
+        (list "" nil)
+        (list (display-to-string (first args)) (rest args))))
+    (define
+      (consume-num args radix)
+      (if
+        (nil? args)
+        (list "" nil)
+        (list (number->string (first args) radix) (rest args))))
+    (define
+      (loop i args)
+      (cond
+        ((>= i n) (string-buffer->string buf))
+        ((and (= (substring template i (+ i 1)) "~") (< (+ i 1) n))
+          (let
+            ((dir (substring template (+ i 1) (+ i 2))))
+            (cond
+              ((= dir "a")
+                (let
+                  ((p (consume-arg args)))
+                  (do
+                    (string-buffer-append! buf (first p))
+                    (loop (+ i 2) (first (rest p))))))
+              ((= dir "s")
+                (if
+                  (nil? args)
+                  (loop (+ i 2) args)
+                  (do
+                    (string-buffer-append!
+                      buf
+                      (write-to-string (first args)))
+                    (loop (+ i 2) (rest args)))))
+              ((= dir "d")
+                (let
+                  ((p (consume-num args 10)))
+                  (do
+                    (string-buffer-append! buf (first p))
+                    (loop (+ i 2) (first (rest p))))))
+              ((= dir "x")
+                (let
+                  ((p (consume-num args 16)))
+                  (do
+                    (string-buffer-append! buf (first p))
+                    (loop (+ i 2) (first (rest p))))))
+              ((= dir "o")
+                (let
+                  ((p (consume-num args 8)))
+                  (do
+                    (string-buffer-append! buf (first p))
+                    (loop (+ i 2) (first (rest p))))))
+              ((= dir "b")
+                (let
+                  ((p (consume-num args 2)))
+                  (do
+                    (string-buffer-append! buf (first p))
+                    (loop (+ i 2) (first (rest p))))))
+              ((= dir "f")
+                (if
+                  (nil? args)
+                  (loop (+ i 2) args)
+                  (do
+                    (string-buffer-append!
+                      buf
+                      (format-decimal (first args) 6))
+                    (loop (+ i 2) (rest args)))))
+              ((= dir "%")
+                (do
+                  (string-buffer-append! buf "\n")
+                  (loop (+ i 2) args)))
+              ((= dir "&")
+                (do
+                  (let
+                    ((so-far (string-buffer->string buf)))
+                    (when
+                      (or
+                        (= (string-length so-far) 0)
+                        (not
+                          (=
+                            (substring
+                              so-far
+                              (- (string-length so-far) 1)
+                              (string-length so-far))
+                            "\n")))
+                      (string-buffer-append! buf "\n")))
+                  (loop (+ i 2) args)))
+              ((= dir "~")
+                (do
+                  (string-buffer-append! buf "~")
+                  (loop (+ i 2) args)))
+              ((= dir "t")
+                (do
+                  (string-buffer-append! buf "\t")
+                  (loop (+ i 2) args)))
+              (else
+                (do
+                  (string-buffer-append! buf "~")
+                  (loop (+ i 1) args))))))
+        (else
+          (do
+            (string-buffer-append!
+              buf
+              (substring template i (+ i 1)))
+            (loop (+ i 1) args)))))
+    (loop 0 args)))