feat(test): ludic test -j N runs tests side by side

Every file is compiled, linked and every test block run as up to N jobs
at once (xargs -P; default the CPU count), each test with a TMPDIR of
its own, and the report is read back in file order. Windows keeps the
sequential path.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/ludic-cli/prelude.ludic

@@ -458,3 +458,13 @@ function cc() -> pointer {
   }
   return "clang -Wno-override-module"
 }
+# the leading decimal digits of `s` as an int (0 for none)
+function s_to_int(s: pointer) -> int {
+  var v = 0
+  var i = 0
+  while s[i] >= 48 and s[i] <= 57 {
+    v = v * 10 + (s[i] - 48)
+    i += 1
+  }
+  return v
+}