spec: string-buffer primitive — make-string-buffer/append!/->string/length

OCaml: StringBuffer of Buffer.t in sx_types.ml; 5 primitives in
sx_primitives.ml (make-string-buffer, string-buffer?, string-buffer-append!,
string-buffer->string, string-buffer-length); inspect case added.

JS: SxStringBuffer with array+join backend; _string_buffer marker for
typeOf dispatch and dict? exclusion (also excludes _vector from dict?).

spec/primitives.sx: 5 define-primitive entries.
17/17 tests pass on both OCaml and JS.

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

spec/tests/test-string-buffer.sx

@@ -0,0 +1,131 @@
+(defsuite
+  "string-buffer"
+  (deftest
+    "make-string-buffer creates a string-buffer"
+    (let ((buf (make-string-buffer))) (assert (string-buffer? buf))))
+  (deftest
+    "string-buffer? is false for non-buffers"
+    (assert= false (string-buffer? "hello"))
+    (assert= false (string-buffer? 42))
+    (assert= false (string-buffer? nil))
+    (assert= false (string-buffer? (list)))
+    (assert= false (string-buffer? {:key "val"})))
+  (deftest
+    "type-of returns string-buffer"
+    (assert= "string-buffer" (type-of (make-string-buffer))))
+  (deftest
+    "empty buffer converts to empty string"
+    (let
+      ((buf (make-string-buffer)))
+      (assert= "" (string-buffer->string buf))))
+  (deftest
+    "empty buffer has length zero"
+    (let
+      ((buf (make-string-buffer)))
+      (assert= 0 (string-buffer-length buf))))
+  (deftest
+    "single append accumulates string"
+    (let
+      ((buf (make-string-buffer)))
+      (string-buffer-append! buf "hello")
+      (assert= "hello" (string-buffer->string buf))))
+  (deftest
+    "multiple appends join in order"
+    (let
+      ((buf (make-string-buffer)))
+      (string-buffer-append! buf "foo")
+      (string-buffer-append! buf "bar")
+      (string-buffer-append! buf "baz")
+      (assert= "foobarbaz" (string-buffer->string buf))))
+  (deftest
+    "length tracks total bytes appended"
+    (let
+      ((buf (make-string-buffer)))
+      (string-buffer-append! buf "abc")
+      (string-buffer-append! buf "de")
+      (assert= 5 (string-buffer-length buf))))
+  (deftest
+    "append returns nil"
+    (let
+      ((buf (make-string-buffer)))
+      (assert= nil (string-buffer-append! buf "x"))))
+  (deftest
+    "appending empty string is harmless"
+    (let
+      ((buf (make-string-buffer)))
+      (string-buffer-append! buf "start")
+      (string-buffer-append! buf "")
+      (string-buffer-append! buf "end")
+      (assert= "startend" (string-buffer->string buf))
+      (assert= 8 (string-buffer-length buf))))
+  (deftest
+    "buffer is still usable after string-buffer->string"
+    (let
+      ((buf (make-string-buffer)))
+      (string-buffer-append! buf "hello")
+      (string-buffer->string buf)
+      (string-buffer-append! buf " world")
+      (assert= "hello world" (string-buffer->string buf))))
+  (deftest
+    "two buffers are independent"
+    (let
+      ((b1 (make-string-buffer)) (b2 (make-string-buffer)))
+      (string-buffer-append! b1 "one")
+      (string-buffer-append! b2 "two")
+      (string-buffer-append! b1 "ONE")
+      (assert= "oneONE" (string-buffer->string b1))
+      (assert= "two" (string-buffer->string b2))))
+  (deftest
+    "loop building — linear string concat"
+    (let
+      ((buf (make-string-buffer)))
+      (let
+        loop
+        ((i 0))
+        (when
+          (< i 5)
+          (string-buffer-append! buf (str i))
+          (loop (+ i 1))))
+      (assert= "01234" (string-buffer->string buf))
+      (assert= 5 (string-buffer-length buf))))
+  (deftest
+    "building CSV row with separator"
+    (let
+      ((buf (make-string-buffer)) (items (list "a" "b" "c" "d")))
+      (let
+        loop
+        ((remaining items) (is-first true))
+        (when
+          (not (empty? remaining))
+          (when (not is-first) (string-buffer-append! buf ","))
+          (string-buffer-append! buf (first remaining))
+          (loop (rest remaining) false)))
+      (assert= "a,b,c,d" (string-buffer->string buf))))
+  (deftest
+    "unicode characters accumulate correctly"
+    (let
+      ((buf (make-string-buffer)))
+      (string-buffer-append! buf "こんにちは")
+      (string-buffer-append! buf " ")
+      (string-buffer-append! buf "世界")
+      (assert= "こんにちは 世界" (string-buffer->string buf))))
+  (deftest
+    "repeated to-string calls are consistent"
+    (let
+      ((buf (make-string-buffer)))
+      (string-buffer-append! buf "test")
+      (assert= (string-buffer->string buf) (string-buffer->string buf))))
+  (deftest
+    "building with join pattern produces correct output"
+    (let
+      ((buf (make-string-buffer))
+        (words (list "the" "quick" "brown" "fox")))
+      (let
+        loop
+        ((remaining words) (sep ""))
+        (when
+          (not (empty? remaining))
+          (string-buffer-append! buf sep)
+          (string-buffer-append! buf (first remaining))
+          (loop (rest remaining) " ")))
+      (assert= "the quick brown fox" (string-buffer->string buf)))))