fix: copy per-chunk buffers and final tcm to avoid iterator-buffer reuse

[gipert]

Co-authored-by: aider (gpt-5.2) [email protected]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.56%. Comparing base (a22f667) to head (f0a73af).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #654   +/-   ##
=======================================
  Coverage   52.56%   52.56%           
=======================================
  Files          60       60           
  Lines        8274     8275    +1     
=======================================
+ Hits         4349     4350    +1     
  Misses       3925     3925           

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