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>

hosts/javascript/run_tests.js

@@ -344,7 +344,7 @@ if (fs.existsSync(swapPath)) {
 }
 
 // Load spec library files (define-library modules imported by tests)
-for (const libFile of ["signals.sx", "coroutines.sx"]) {
+for (const libFile of ["stdlib.sx", "signals.sx", "coroutines.sx"]) {
   const libPath = path.join(projectDir, "spec", libFile);
   if (fs.existsSync(libPath)) {
     const libSrc = fs.readFileSync(libPath, "utf8");