Phase 8b: modern names for the raw-memory and OS/IO primitives

Groups B and C of the leftover-primitive cleanup — renames, not new machinery,
and deliberately NO unsafe_ prefix (a __-prefix is itself a C convention, and an
`unsafe` marker carries no signal in a fully-manual-memory language with no safe
subset to contrast against).

  memory:  mem_free -> free   mem_realloc -> resize   mem_set -> fill
           ptr_add -> offset
  process: os_argc -> arg_count   os_arg -> arg   os_exit -> exit
           os_system -> run   os_getenv -> getenv   read_byte -> read_char
  dead:    mem_copy, os_time, write_byte (0 uses) — deleted

Two reseeds: accept both old and new names in the intrinsic dispatch, then
migrate every call site and drop the old names. file_open/read/write/seek/tell/
close are left as-is — they're the domain-prefixed syscall layer wrapped by
read_file, not the argc/argv-style C-ness the audit targeted; a `File` type is a
separate, larger design if wanted.

test.sh's CLI smoke updated (os_exit -> exit); check-vocabulary's grammar marker
moved off the deleted names. Reseeded (22243 lines); C-free fixpoint holds;
goldens identical; 18/18; vocab + doc-fences clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

selfhost/parse.ludic

@@ -17,7 +17,7 @@ fn perr(msg: ptr) -> void {
   file_write(e, "ludicc(self): parse error: ", 27)
   file_write(e, msg, len(msg))
   file_write(e, "\n", 1)
-  os_exit(1)
+  exit(1)
 }
 
 fn eat_op(v: ptr) -> void { if not is_op(v) { perr(v) }; pi = pi + 1 }