refactor(assemble): split built-in assemblers by physics

[kilickursat]

Summary

This PR refactors the built-in assembler layout from the monolithic
tensormesh/assemble/builtin.py module into a builtin/ package organized by
physics category.

The new hierarchy is:

tensormesh/assemble/builtin/
├── common.py
├── fluid/
├── solid/
└── electromagnetic/

Geomechanics lives under the solid family:

tensormesh/assemble/builtin/solid/geomechanics.py

Scope

• Splits built-in assemblers by physics category
• Moves Laplace and mass assemblers under builtin/fluid/
• Moves linear elasticity, Neo-Hookean, J2 plasticity, Drucker-Prager plasticity, and contact under builtin/solid/
• Adds a reserved builtin/electromagnetic/ namespace
• Preserves existing public imports from tensormesh.assemble
• Preserves imports from tensormesh.assemble.builtin
• Adds category-level imports such as tensormesh.assemble.builtin.solid
• Adds import-compatibility tests

Out of scope

• No numerical behavior changes
• No Drucker-Prager formula changes
• No J2Plasticity behavior changes
• No public API behavior changes
• No tunnel-specific API
• No .po translation updates

Validation

Ran locally:

• pytest tests/assemble/test_builtin_imports.py -v
• pytest tests/assemble -v
• pytest tests/functional/test_drucker_prager.py -v
• pytest tests/material/test_frictional_material.py -v
• pytest tests/assemble/test_drucker_prager_plasticity.py -v
• pytest tests/assemble/test_drucker_prager_triaxial_example.py -v
• pytest tests/assemble/test_drucker_prager_footing_example.py -v
• python examples/solid/geomechanics/drucker_prager_triaxial/drucker_prager_triaxial.py --no-plot --steps 16
• python examples/solid/geomechanics/drucker_prager_footing/drucker_prager_footing.py --no-plot --steps 6 --chara-length 0.60
• cd docs && make html
• cd docs && make zh

Also verified import compatibility for:

from tensormesh.assemble import J2Plasticity
from tensormesh.assemble import DruckerPragerPlasticity
from tensormesh.assemble.builtin import J2Plasticity
from tensormesh.assemble.builtin.solid import DruckerPragerPlasticity

Notes

This is a structure-only refactor. The previous public import paths are preserved
through builtin/__init__.py and tensormesh/assemble/__init__.py.