Adjust FMG approximation - Skip costly multigrid cycles on finest level

[julianlitz]

During FMG initialization, multigrid cycles were being applied at every level including the finest (depth 0). However, any cycles applied at depth 0 during FMG are redundant and should be counted towards the standard multigrid iterations anyway.

New behavior: The initial FMG approximation now ends with the interpolation to the finest grid. No further MGC is applied.

Why this matters for PCG

This becomes especially important when combining FMG with a Preconditioned Conjugate Gradient (PCG) solver. Any multigrid cycles burned at depth 0 during FMG are slow and expensive (finest grid = largest problem), and they delay the point at which PCG can take over with its faster convergence. By skipping these redundant cycles, PCG starts earlier, reducing the total iteration count and overall solve time.

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[codecov]

Codecov Report

❌ Patch coverage is 76.66667% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.21%. Comparing base (39ce1dd) to head (d0be022).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/GMGPolar/solver.cpp	74.07%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #185      +/-   ##
==========================================
+ Coverage   95.18%   95.21%   +0.03%     
==========================================
  Files          94       94              
  Lines        9345     9327      -18     
==========================================
- Hits         8895     8881      -14     
+ Misses        450      446       -4     

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