Fix contains? primitive to handle strings in both JS and Python

The JS contains? used `k in c` which throws TypeError on strings.
The Python version silently returned False for strings. Both now
use indexOf/`in` for substring matching on strings.

Fixes: sx.js MOUNT PARSE ERROR on blog index where
(contains? current-local-href "?") was evaluated client-side.

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

shared/sx/primitives.py

@@ -196,6 +196,8 @@ def prim_is_empty(coll: Any) -> bool:
 
 @register_primitive("contains?")
 def prim_contains(coll: Any, key: Any) -> bool:
+    if isinstance(coll, str):
+        return str(key) in coll
     if isinstance(coll, dict):
         k = key.name if isinstance(key, Keyword) else key
         return k in coll