[experimental] Curved-element meshing and Palace 3D photonics example

[mdmaas]

This PR implements spline-fitting to replace curve loops, which enables curved-element meshing.

This is potentially interesting for several PDKs with curved features (like QPDK) but maybe most interesting for 3D photonics, for which Palace could lead to better performance than Meep.

[gemini-code-assist]

Code Review

This pull request introduces support for patterned dielectric layers and high-order geometric mesh elements in Palace simulations. Key features include the ability to disable synthetic dielectric regions, the implementation of spline and bspline curve fitting for dielectric boundaries, and the integration of high-order elements into the meshing and visualization pipelines. A new example notebook for a directional coupler is provided; however, feedback indicates that the current validation logic incorrectly triggers a ValueError for purely dielectric simulations, which is a critical issue for photonic component modeling.

[gemini-code-assist]

This cell fails with a ValueError during mesh validation, specifically complaining about the absence of conductor surfaces. This seems to indicate that the validation logic in PalaceSimMixin.validate_mesh() doesn't correctly handle purely dielectric simulations, which is common for photonic components like this directional coupler.

Since this notebook serves as an example for the new curved-element meshing feature, it's critical that it runs successfully. The validation should likely be relaxed for simulations that only use wave ports on dielectric materials.

[joamatab]

LGTM - curved-element meshing and Palace 3D photonics

[codecov]

Codecov Report

❌ Patch coverage is 50.94851% with 181 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.25%. Comparing base (710d2c2) to head (d3773c5).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gsim/palace/base.py	24.67%	48 Missing and 10 partials ⚠️
src/gsim/viz.py	14.54%	44 Missing and 3 partials ⚠️
src/gsim/palace/mesh/geometry.py	61.76%	16 Missing and 10 partials ⚠️
src/gsim/palace/mesh/gmsh_utils.py	73.03%	13 Missing and 11 partials ⚠️
src/gsim/palace/mesh/generator.py	51.11%	18 Missing and 4 partials ⚠️
src/gsim/palace/mesh/config_generator.py	62.50%	1 Missing and 2 partials ⚠️
src/gsim/common/stack/extractor.py	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
+ Coverage   56.18%   56.25%   +0.06%     
==========================================
  Files          59       59              
  Lines        6966     7361     +395     
  Branches     1270     1386     +116     
==========================================
+ Hits         3914     4141     +227     
- Misses       2642     2764     +122     
- Partials      410      456      +46     

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