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>

selfhost/emit_addr.ludic

@@ -21,7 +21,12 @@ fn emit_member_addr(e: Node) -> ptr {
 # address of `base[index]` (slices only in this subset)
 fn emit_index_addr(e: Node) -> ptr {
   let base = emit_expr(e.a)
-  if not is_slice_ty(base.ty) { perr("indexing a non-slice") }
+  if not is_slice_ty(base.ty) {                   # a raw ptr/str: address of byte i.
+    let bi = emit_expr(e.b)                        # (evaluate index first — it may set g_addr_ty)
+    let r = emit_bind(`getelementptr inbounds i8, ptr {base.code}, i32 {bi.code}`)
+    g_addr_ty = "byte"                             # set last so the caller sees it
+    return r
+  }
   let el = slice_elem(base.ty)
   g_addr_ty = el
   # load the data pointer from the slice header (field 0)