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>

sx/content/pages.py

@@ -209,7 +209,7 @@ PRIMITIVES = {
     "Arithmetic": ["+", "-", "*", "/", "mod", "sqrt", "pow", "abs", "floor", "ceil", "round", "min", "max"],
     "Comparison": ["=", "!=", "<", ">", "<=", ">="],
     "Logic": ["not", "and", "or"],
-    "String": ["str", "upper", "lower", "trim", "split", "join", "starts-with?", "ends-with?", "replace", "substring"],
+    "String": ["str", "upper", "lower", "trim", "split", "join", "index-of", "starts-with?", "ends-with?", "replace", "substring"],
     "Collections": ["list", "dict", "len", "first", "last", "rest", "nth", "cons", "append", "keys", "vals", "merge", "assoc", "range", "concat", "reverse", "sort", "flatten", "zip"],
     "Higher-Order": ["map", "map-indexed", "filter", "reduce", "some", "every?", "for-each"],
     "Predicates": ["nil?", "number?", "string?", "list?", "dict?", "empty?", "contains?", "odd?", "even?", "zero?"],