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>

examples/strings.ludic

@@ -22,5 +22,10 @@ program Strings {
     let n = 42
     if `{greeting}, {who}!` == "hello, Ludic!" { print(6) }
     if `n={n}, next={n + 1}` == "n=42, next=43" { print(7) }
+
+    # slicing: s[a..b] is the substring of bytes [a, b), and len(s) its length
+    let full = "hello world"
+    if full[0..5] == "hello" { print(8) }
+    if full[6..len(full)] == "world" { print(9) }
   }
 }