feat: Use output surfaces in internal nodes

[effigies]

Small cleanup. We generally follow the convention of using files in the output directory in our buffers for reuse to ensure more consistency between first runs and reruns with precomputed derivatives. The t1w_buffer is an exception due to the complexity of that logic. There's no reason for the surfaces_buffer to be an exception.

This should resolve the error in #482.

Took the opportunity to mass change # fmt:off/on blocks into # fmt:skips

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.06%. Comparing base (d29e49c) to head (7a4ce31).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #483      +/-   ##
==========================================
+ Coverage   70.82%   75.06%   +4.23%     
==========================================
  Files          25       25              
  Lines        2029     2029              
  Branches      264      264              
==========================================
+ Hits         1437     1523      +86     
+ Misses        537      441      -96     
- Partials       55       65      +10     

Flag	Coverage Δ
ds054	45.60% <ø> (?)

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