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/main.ludic

@@ -18,7 +18,7 @@
 fn base_name(path: ptr) -> ptr {
   var last = 0 - 1
   var i = 0
-  while peek8(path, i) != 0 { if peek8(path, i) == 47 { last = i };  i = i + 1 }
+  while path[i] != 0 { if path[i] == 47 { last = i };  i = i + 1 }
   return path[last + 1..i]
 }
 
@@ -42,7 +42,7 @@ fn getenv_or(name: ptr, dflt: ptr) -> ptr {
 fn ensure_slash(d: ptr) -> ptr {
   let n = len(d)
   if n == 0 { return d }
-  if peek8(d, n - 1) == 47 { return d }
+  if d[n - 1] == 47 { return d }
   return (d + ("/"))
 }
 
@@ -76,7 +76,7 @@ entry {
     else {
       # an unknown flag is ignored (with a note) rather than mistaken for the
       # input file — '-' is ASCII 45
-      if peek8(a, 0) == 45 {
+      if a[0] == 45 {
         let m = `ludicc: ignoring unknown flag {a}\n`
         file_write(file_stderr(), m, len(m))
       } else { path = a }