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/primitives.sx

@@ -798,4 +798,10 @@
   :returns "list"
   :doc "Parse SX source string into a list of AST expressions.")
 
+(define-primitive
+  "make-string-buffer"
+  :params ()
+  :returns "string-buffer"
+  :doc "Create a new empty mutable string buffer for O(1) amortised append.")
+
 (define-module :stdlib.coroutines)