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/emit_core.ludic

@@ -57,7 +57,7 @@ fn llty(t: ptr) -> ptr {
 }
 
 fn is_slice_ty(t: ptr) -> bool { return peek8(t, 0) == 91 and peek8(t, 1) == 93 }   # "[]"
-fn slice_elem(t: ptr) -> ptr { return substr(t, 2, slen(t) - 2) }
+fn slice_elem(t: ptr) -> ptr { return substr(t, 2, len(t) - 2) }
 
 fn find_arch(name: ptr) -> Node {
   var i = 0