Flat_reconstruction assembly classes

[szvsw]

Refactor flat_constructions modules to use class-based assembly strategies.

This refactor moves construction layup logic from conditional blocks into polymorphic builder subclasses, making it easier to customize layups for each assembly type. STRUCTURAL_TEMPLATES are preserved for backward compatibility.

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[cursor]

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Codecov Report

❌ Patch coverage is 89.36170% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.8%. Comparing base (a136df7) to head (92f34a6).

Files with missing lines	Patch %	Lines
epinterface/sbem/flat_constructions/roofs.py	86.4%	8 Missing and 5 partials ⚠️
epinterface/sbem/flat_constructions/walls.py	90.6%	5 Missing and 5 partials ⚠️
epinterface/sbem/flat_constructions/slabs.py	91.1%	4 Missing and 3 partials ⚠️
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Additional details and impacted files

@@                            Coverage Diff                             @@
##           cursor/semi-flat-wall-constructions-2cb3     #62     +/-   ##
==========================================================================
- Coverage                                      67.0%   66.8%   -0.3%     
==========================================================================
  Files                                            42      42             
  Lines                                          5518    5574     +56     
  Branches                                        512     514      +2     
==========================================================================
+ Hits                                           3701    3725     +24     
- Misses                                         1651    1680     +29     
- Partials                                        166     169      +3     

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