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_decl.ludic

@@ -96,14 +96,14 @@ fn ir_flush(path: ptr) -> bool {
   let c = buf_str(code)
   if (path == null) {                             # raw IR to stdout (no trailing newline)
     let out = file_stdout()
-    file_write(out, h, slen(h))
-    file_write(out, c, slen(c))
+    file_write(out, h, len(h))
+    file_write(out, c, len(c))
     return true
   }
   let f = file_open(path, "wb")
   if (f == null) { return false }
-  file_write(f, h, slen(h))
-  file_write(f, c, slen(c))
+  file_write(f, h, len(h))
+  file_write(f, c, len(c))
   file_close(f)
   return true
 }