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/tests/match_bits.ludic

@@ -9,7 +9,7 @@ program T {
     print(classify(90))          # 9
     print((1 << 4))             # 16
     print(((4 | 1) & 6))    # 4
-    let p = mem_alloc(16)
+    let p = bytes(16)
     p[1] = 9999
     print(p[1])          # 9999
   }