Add index-of string primitive: spec, Python, JS, rebootstrap

(index-of s needle from?) returns first index of needle in s, or -1.
Optional start offset. Specced in primitives.sx, implemented in both
hand-written primitives.py and bootstrapper templates, rebootstrapped.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/primitives.sx

@@ -307,6 +307,11 @@
   :returns "any"
   :doc "Slice a string or list from start to end (exclusive). End is optional.")
 
+(define-primitive "index-of"
+  :params (s needle &rest from)
+  :returns "number"
+  :doc "Index of first occurrence of needle in s, or -1 if not found. Optional start index.")
+
 (define-primitive "starts-with?"
   :params (s prefix)
   :returns "boolean"