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>

LANGUAGE.md

@@ -640,7 +640,8 @@ literal; test any pointer/record/slice with `x == null` / `x != null` (an unset
 # control   quit()   print(x)          (a value + newline)
 # convert   str(x) -> str            (int/bool/fixed -> text)
 # length    len(x) -> int            (elements of a slice, or bytes of a string)
-# process   os_argc()->int   os_arg(i)->str        (the command line; argv[0] included)
+# process   arg_count()->int   arg(i)->str          (the command line; argv[0] included)
+#           exit(code)   run(cmd)   getenv(name)   read_char()->int
 #           file_stderr()->ptr  file_stdout()->ptr (handles for file_write)
 ```