Phase 7i: byte indexing p[i] / p[i] = v (retire peek8/poke8)

Raw byte access is now indexing, not C-style peek/poke:

  peek8(src, i)          -> src[i]        (reads a byte, widened to int)
  poke8(out, j, r)       -> out[j] = r    (narrows the int to a byte)

E_INDEX on a non-slice pointer/string lowers to a `getelementptr i8` + load/zext
(read) or trunc/store (write) — byte-identical to the old peek8/poke8, so the
migration reproduces the compiler exactly. A "byte" element type (llty i8) drives
the widen/narrow. The compiler's own byte work now reads naturally, e.g.
`is_slice_ty` is `t[0] == 91 and t[1] == 93`.

Subtlety fixed on the way: g_addr_ty (the out-param carrying the indexed element
type) must be set AFTER evaluating the index expression, since a member/index in
the index would otherwise clobber it — doing it early made a byte read load a
full pointer from a byte address and crash the self-compile.

Two reseeds: add byte-index support keeping peek8/poke8, then migrate 148 call
sites and delete the intrinsics (+ the now-dead emit_gep_i8). Vocabulary drops
peek8/poke8. Reseeded (22604 lines); C-free fixpoint holds; goldens identical;
18/18; vocab clean.

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

runtime/native/ui.ludic

@@ -197,7 +197,7 @@ fn ui_tw(font: int, s: ptr, size: int) -> int {
     if font < tt_n { return rt_text_w(font, s, size) }
   }
   var n = 0
-  while peek8(s, n) != 0 { n = n + 1 }
+  while s[n] != 0 { n = n + 1 }
   return n * 6 * max(size / 8, 1)
 }