Phase 7f: len(x) works on strings too (retire slen)

`len` is now polymorphic — a slice's element count OR a string's byte length —
so the C-style `slen` (strlen) is gone: `slen(name)` -> `len(name)`. emit_len
branches on the operand type (slice header vs @strlen). Two reseeds: add the
string branch, then migrate the 19 slen calls and delete slen.

Reseeded (22565 lines); C-free fixpoint holds; goldens identical; 18/18; vocab +
doc-fences clean. (String slicing s[a..b] to replace substr is deferred to its
own phase.)

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

selfhost/lex.ludic

@@ -41,7 +41,7 @@ fn lex(src: ptr) -> void {
   toks = new []Tok
   var i = 0
   var line = 1
-  let n = slen(src)
+  let n = len(src)
   while i < n {
     let c = peek8(src, i)
     if c == 10 { tok_push(TK_NL, null, 0, line); line = line + 1; i = i + 1; continue }