Fix __notes__ from outermost exception shown on all exceptions in chain

[veeceey]

Summary

• Fixes [BUG] Traceback __notes__ from outermost exception shown on all exceptions in chain #3960 -- Traceback.__notes__ from the outermost exception was incorrectly displayed on every exception in a chained traceback.
• Moved the notes assignment (getattr(exc_value, "__notes__", ...)) from before the while True loop to inside it, so each Stack reads notes from its own exc_value rather than reusing the same list for all.
• Added two regression tests: one verifying notes only appear on the exception that has them, and another verifying each exception in a chain gets its own distinct notes.

Root Cause

In Traceback.extract(), the notes list was read once from the initial exc_value before the loop that walks __cause__/__context__. Since the same notes reference was passed to every Stack, all exceptions in the chain incorrectly displayed the outermost exception's notes.

Test plan

• Verified with the exact reproduction script from the issue -- note now only appears on RuntimeError, not ValueError
• Added test_notes_chained_exceptions -- notes on outer only, inner has none
• Added test_notes_on_each_chained_exception -- each exception has its own distinct notes
• All 24 existing traceback tests pass (1 skipped)

🤖 Generated with Claude Code