Phase 7h: string slicing s[a..b] (retire substr)

`s[a..b]` is a fresh substring of the bytes [a, b) — the modern, end-based form
of the C-style `substr(s, start, count)`:

  substr(src, start, i - start)   -> src[start..i]
  substr(t, 2, len(t) - 2)        -> t[2..len(t)]
  substr(src, i, 2)               -> src[i..i + 2]

Mechanics: a new E_SLICE postfix (`base[lo..hi]`, distinct from `base[i]`
indexing) lowers to a @fn_str_slice prelude (malloc + copy + terminate), emitted
once into any program that slices. Two reseeds: add the syntax + prelude, then
migrate the 22 substr calls and delete substr. The migrator recognises the
common `count == end - start` shape and emits the clean `s[start..end]` rather
than `s[start..start + (end - start)]`.

examples/strings.ludic gains slicing (now prints 1..9). Reseeded (22673 lines);
C-free fixpoint holds; goldens identical; 18/18; vocab + doc-fences clean.

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

selfhost/emit_ui.ludic

@@ -41,7 +41,7 @@ fn has_ui() -> bool {
 
 # index of a UI_<name>: a ui block's root, or a widget's id=
 fn ui_index_of(nm: ptr) -> int {
-  let s = substr(nm, 3, len(nm) - 3)          # strip "UI_"
+  let s = nm[3..len(nm)]          # strip "UI_"
   let u = 0;  var bi = 0
   var i = 0
   while i < len(prog) {