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/primitives.py

@@ -299,6 +299,10 @@ def prim_slice(coll: Any, start: int, end: Any = None) -> Any:
         return coll[start:]
     return coll[start:int(end)]
 
+@register_primitive("index-of")
+def prim_index_of(s: str, needle: str, start: int = 0) -> int:
+    return str(s).find(needle, int(start))
+
 @register_primitive("starts-with?")
 def prim_starts_with(s, prefix: str) -> bool:
     if not isinstance(s, str):