Zero-copy parse, eliminate string copies, optimize hot paths

Performance optimizations addressing PyO3 overhead analysis:

1. Zero-copy parse for bytes input (#6): DocumentOwner enum uses
   PyBackedBytes to borrow directly from Python bytes object's
   internal buffer, avoiding a full memcpy of the XML document.
   str input still copies (Python str -> UTF-8 encoding required).

2. Eliminate String intermediaries (#4): All text-returning methods
   (xpath_text, xpath_string, .text, .tail, .get, .keys, .items,
   itertext, text_content, tostring) now return Py<PyString> built
   directly from &str slices. Skips Rust String allocation that
   PyO3 would then copy again into Python.

3. interned_tag_fast (#3): Hot paths (child_tags, descendant_tags,
   make_element_borrowed, make_elements) now accept &IndexWithMeta
   directly, avoiding redundant borrow_dependent() calls in tight
   loops.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: cigrainger

python/simdxml/_core.pyi

@@ -166,8 +166,8 @@ class CompiledXPath:
 def parse(data: bytes | str) -> Document:
     """Parse XML into a Document.
 
-    Accepts ``bytes`` or ``str``. Returns a Document that can be queried
-    with XPath or traversed element-by-element.
+    Accepts ``bytes`` or ``str``. For bytes input, the buffer is used
+    directly (zero-copy). For str input, the string is encoded to UTF-8.
     """
     ...