Bugfix: Fix issue 1951 half face neighbor order

[holke]

Describe your changes here:

Fixes #1951
The half face neighbor computation did not compute the neighbors in linear Id order.
We fix this by computing the neighbors via

Compute same level neighbor.
Compute the children at face.

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[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.65%. Comparing base (23dba8f) to head (37cb57d).
⚠️ Report is 32 commits behind head on main.

Files with missing lines	Patch %	Lines
src/t8_forest/t8_forest.cxx	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1952      +/-   ##
==========================================
- Coverage   75.67%   75.65%   -0.02%     
==========================================
  Files         105      105              
  Lines       18648    18641       -7     
==========================================
- Hits        14111    14103       -8     
- Misses       4537     4538       +1     

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