Cache S-matrix linear-algebra artifacts for repeated solves; add docs and tests

[darkviruzz]

Motivation

• Reduce redundant linear-algebra cost in repeated S-matrix assembly by caching per-layer inverses/factorizations.
• Provide internal invalidation rules so cached artifacts stay consistent when layer eigensystems or material grids change.
• Document planned optimization tasks (1D mode inference, S-matrix caching, phased implementation) for future performance work.

Description

• Added an internal cache on the solver object: _smatrix_cache, with helper methods _invalidate_smatrix_cache() and _get_smatrix_cache() that build and return cached phi and kp@phi inverses.
• Wire the cache into the solve path by changing GetSMatrix, SolveExterior, and SolveInterior to accept an optional smatrix_cache parameter and use cached phi/kpphi inverses when provided.
• Invalidate the cache at appropriate points by calling _invalidate_smatrix_cache() from setup and when patterned-layer epsilon is updated (added call in GridLayer_geteps and after initial layer setup).
• Make public solve entry points use the cache by passing self._get_smatrix_cache() from methods like RT_Solve, GetAmplitudes*, and Volume_integral.
• Add documentation docs/optimization_tasks.rst describing 1D inference and S-matrix caching tasks and include it in docs/index.rst.
• Add unit tests in tests/test_rcwa.py covering cache availability, cached vs uncached numerical equivalence for GetSMatrix, SolveExterior, and SolveInterior, cache reuse across repeated solves, and cache invalidation after updating patterned epsilon.

Testing

• Ran the test suite (pytest) including the new tests test_smatrix_cache_unavailable_before_patterned_setup, test_getsmatrix_cached_vs_uncached_identical, test_exterior_interior_cached_vs_uncached_identical, and test_smatrix_cache_reuse_and_invalidate, and all tests completed successfully.
• Existing RCWA regression checks in test_rcwa were also executed and remained within tolerance.
• No automated tests failed after these changes.

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