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/bootstrap_py.py

@@ -1705,6 +1705,7 @@ PRIMITIVES["trim"] = lambda s: str(s).strip()
 PRIMITIVES["split"] = lambda s, sep=" ": str(s).split(sep)
 PRIMITIVES["join"] = lambda sep, coll: sep.join(coll)
 PRIMITIVES["replace"] = lambda s, old, new: s.replace(old, new)
+PRIMITIVES["index-of"] = lambda s, needle, start=0: str(s).find(needle, start)
 PRIMITIVES["starts-with?"] = lambda s, p: str(s).startswith(p)
 PRIMITIVES["ends-with?"] = lambda s, p: str(s).endswith(p)
 PRIMITIVES["slice"] = lambda c, a, b=None: c[a:b] if b is not None else c[a:]