Stabilize _ast_unparse.py and F-string/Formatting Path #23

Summary

Background:
_ast_unparse.py and its helpers (_str_literal_helper, _ftstring_helper,
_write_ftstring_inner) show dense mismatches, likely due to shared expression and
formatting pipelines.

Task:

• Focus on ast-unparse and formatting helpers:
  • Compare RustPython and CPython emission around return/except paths and control edges.
  • Fix instruction order and jump target normalization in these code paths.
• Prefer a minimal diff strategy: align stack and control behavior first, then optimize.

Acceptance criteria:

• Notable reduction in mismatch clusters for all _Unparser.* functions listed in the
  report.
• _ast_unparse.py aggregate mismatch no longer dominates report when running summary
  over Lib (excluding test/vendor files).

Details