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/str.ludic

@@ -2,12 +2,6 @@
 # slicing out substrings, and integer<->text. All over raw NUL-terminated
 # byte buffers reached with peek8/poke8.
 
-fn slen(s: ptr) -> int {
-  var n = 0
-  while peek8(s, n) != 0 { n = n + 1 }
-  return n
-}
-
 # a fresh NUL-terminated copy of src[start .. start+n]
 fn substr(src: ptr, start: int, n: int) -> ptr {
   let b = mem_alloc(n + 1)