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_draw.ludic

@@ -219,16 +219,16 @@ fn rt_ui_set_text(id: int, s: str) -> void {
   if id >= ui_n { return }
   let base = id * 96
   var i = 0
-  var ch = peek8(s, 0)
+  var ch = s[0]
   while ch != 0 {
     if i >= 95 { ch = 0 }
     if ch != 0 {
-      poke8(ui_dyn, base + i, ch)
+      ui_dyn[base + i] = ch
       i = i + 1
-      ch = peek8(s, i)
+      ch = s[i]
     }
   }
-  poke8(ui_dyn, base + i, 0)
+  ui_dyn[base + i] = 0
   poke32(ui_hasdyn, id, 1)
 }
 
@@ -239,12 +239,12 @@ fn rt_ui_set_int(id: int, n: int) -> void {
   var at = 0
   var v = n
   if v < 0 {
-    poke8(ui_dyn, base, 45)
+    ui_dyn[base] = 45
     at = 1
     v = 0 - v
   }
   if v == 0 {
-    poke8(ui_dyn, base + at, 48)
+    ui_dyn[base + at] = 48
     at = at + 1
   } else {
     var digits = 0
@@ -257,12 +257,12 @@ fn rt_ui_set_int(id: int, n: int) -> void {
     while p > 0 {
       var div = 1
       for k in 1 .. p { div = div * 10 }
-      poke8(ui_dyn, base + at, 48 + (v / div) % 10)
+      ui_dyn[base + at] = 48 + (v / div) % 10
       at = at + 1
       p = p - 1
     }
   }
-  poke8(ui_dyn, base + at, 0)
+  ui_dyn[base + at] = 0
   poke32(ui_hasdyn, id, 1)
 }