Fix parser escape ordering and prim_get for non-dict objects

Parser: chained .replace() calls processed \n before \\, causing \\n
to become a real newline. Replaced with character-by-character
_unescape_string. Fixes 2 parser spec test failures.

Primitives: prim_get only handled dict and list. Objects with .get()
methods (like PageDef) returned None. Added hasattr fallback.
Fixes 9 defpage spec test failures.

All 259 spec tests now pass (was 244/259).

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

shared/sx/primitives.py

@@ -374,6 +374,8 @@ def prim_get(coll: Any, key: Any, default: Any = None) -> Any:
         return default
     if isinstance(coll, list):
         return coll[key] if 0 <= key < len(coll) else default
+    if hasattr(coll, "get"):
+        return coll.get(key, default)
     return default
 
 @register_primitive("len")