Phase 7k: bytes(n) / words(n) allocators (retire mem_alloc)

Allocation reads as intent, not malloc:

  mem_alloc(64)          -> bytes(64)              (64 bytes -> a byte buffer)
  mem_alloc(w * h * 4)   -> words(w * h)           (w*h 32-bit words)

bytes(n) mallocs n bytes and returns a plain pointer (byte-indexed); words(n)
mallocs n*4 bytes and returns a `words` pointer (int-indexed). Since words(X) and
mem_alloc(X*4) allocate the identical number of bytes, the migration cannot change
any allocation size — the `* 4` factor just moves from the argument into the
allocator name, pairing naturally with the Phase-7j `words` retyping
(`var fb: words = words(w * h)`).

Migrated 102 sites (mem_alloc(E*4) -> words(E), else bytes(E)); deleted the
mem_alloc intrinsic. mem_realloc/free/copy/set stay as the low-level
reallocation/free family. Reseeded (22565 lines); C-free fixpoint holds; goldens
byte-identical; 18/18; vocab + doc-fences clean.

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

selfhost/io.ludic

@@ -7,7 +7,7 @@ fn read_file(path: str) -> ptr {
   file_seek(f, 0, 2)
   let n = file_tell(f)
   file_seek(f, 0, 0)
-  let buf = mem_alloc(n + 1)
+  let buf = bytes(n + 1)
   file_read(f, buf, n)
   buf[n] = 0
   file_close(f)