feat(tidy3d): FXC-1539 implement spatially varying heat conductivity

[damiano-flex]

Frontend implementation of spatially varying conductivity.

• Potentially, we could enable spatially varying conductivity for conductors too.
• I am not 100% sure on the plot part. It had to be changed to accommodate the non-uniform heat source, but I am wondering if it pollutes other plots. Let's see if we get something useful out from bug-bot.

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Note

Medium Risk
Expands core simulation/material validation and plotting paths to support non-scalar conductivities; risk is mainly in schema/serialization compatibility and potential plotting regressions (norm/overlays), plus a new unsupported-case guard for semiconductor mixes.

Overview
Adds support for spatially varying thermal conductivity by allowing SolidMedium/SolidSpec.conductivity to accept SpatialDataArray (including from_si_units scaling) and updating JSON schemas to allow non-numeric conductivity values.

Extends Scene.plot_structures_property to plot heat_conductivity/electric_conductivity with updated bounds handling for spatial data and an optional pcolormesh overlay clipped to each structure; includes tests covering acceptance, unit scaling, overlay rendering, and log-scale norm behavior.

Also allows ChargeConductorMedium.conductivity to be a SpatialDataArray, and adds a HeatChargeSimulation setup check that rejects spatial conductor conductivity when semiconductor structures are present (with a corresponding test).

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[github-actions]

Diff Coverage

Diff: origin/develop...HEAD, staged and unstaged changes

• tidy3d/components/material/tcad/charge.py (100%)
• tidy3d/components/material/tcad/heat.py (100%)
• tidy3d/components/scene.py (94.5%): Missing lines 1242,1855-1856,1970
• tidy3d/components/tcad/simulation/heat_charge.py (100%)

Summary

• Total: 91 lines
• Missing: 4 lines
• Coverage: 95%

tidy3d/components/scene.py

Lines 1238-1246

  1238                     ax=ax,
  1239                     norm=norm,
  1240                 )
  1241             elif property == "electric_conductivity":
! 1242                 Scene._add_cbar(
  1243                     vmin=property_min,
  1244                     vmax=property_max,
  1245                     label=f"Electric conductivity ({CONDUCTIVITY})",
  1246                     cmap=STRUCTURE_HEAT_COND_CMAP,

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Lines 1851-1860

  1851             cond_list = []
  1852             for medium in cond_mediums:
  1853                 cond = medium.charge.conductivity
  1854                 if isinstance(cond, SpatialDataArray):
! 1855                     cond_list.append(float(np.min(cond.values)))
! 1856                     cond_list.append(float(np.max(cond.values)))
  1857                 else:
  1858                     cond_list.append(float(cond))
  1859 
  1860         if len(cond_list) == 0:

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Lines 1966-1974

  1966         elif property == "electric_conductivity":
  1967             if isinstance(medium.charge, ChargeConductorMedium):
  1968                 cond = medium.charge.conductivity
  1969                 if isinstance(cond, SpatialDataArray):
! 1970                     return cond
  1971         return None
  1972 
  1973     def _plot_spatial_conductivity_on_shape(
  1974         self,

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