Use native AST positions in _parse, drop source-order traversal

[Carreau]

Since Python 3.8 the built-in parser reports correct lineno/col_offset (and end_lineno/end_col_offset) for every node, including multiline string literals, which historically misreported their position as the ending line with col_offset == -1. That made a large chunk of _parse.py obsolete:

• Replace the recursive _annotate_ast_startpos / _annotate_ast_nodes (which re-derived positions to work around the old quirk) with a flat ast.walk that reads native positions directly, keeping the decorator-start special case.
• Delete _flatten_ast_nodes, _iter_child_nodes_in_order, _iter_child_nodes_in_order_internal_1 and _walk_ast_nodes_in_order, including the per-Python-version _fields asserts that broke on each new release.
• Make string_literals / _get_docstring_nodes sort by native startpos instead of relying on the hand-maintained source-order traversal.
• Remove the dead endpos branch in _split_code_lines (.endpos is never set on AST nodes) and the now-unused imports / AstNodeContext.
• _autoimp: import MatchAs from ast directly rather than re-exporting it through _parse.

Net -335 lines in _parse.py with no behavior change on Python >=3.12 (the only version-specific path, the <3.12 f-string clamp, never ran there). All parse/import/autoimp suites pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.97%. Comparing base (ebbe1b4) to head (3828e29).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #535      +/-   ##
==========================================
+ Coverage   89.92%   89.97%   +0.04%     
==========================================
  Files          57       57              
  Lines       17694    17580     -114     
==========================================
- Hits        15912    15818      -94     
+ Misses       1782     1762      -20     

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