refactor: comprehensive SIAC codebase revision with docs

[MarcYin]

Summary

• Deduplicate resampling: Extract canonical geo/resample.py module, eliminating ~470 lines of 3-way duplication across pipeline, corrector, and assembly
• Fix alpha/band_weight_power: Thread algorithms.solver.alpha through MultiGridConfig → CostFunctionConfig so the user's configured value is no longer silently ignored (default aligned to -1.6)
• Performance & correctness: Vectorize Laplacian boundary construction, pre-allocate solver DataArrays to avoid per-iteration allocation, wrap RAA to [0, 2π), add division-by-zero guard in apply_correction
• Simplify internals: Dict-based lazy imports in __init__.py, _ConfigShortcutsMixin to deduplicate ~12 config properties, keyword-arg grid assembler interface replacing fragile inspect.signature dispatch, remove empty Resolved* subclasses
• Documentation: Update 9 doc files — add geo/resample.py to architecture, expand glossary with solver terms (DCT, multi-grid, RAA, RTModelBackend), document cost function and multi-grid strategy, annotate solver config fields, add RTModelBackend protocol and resampling utilities to developer guide

Test plan

• pixi run lint passes
• pixi run test-fast unit tests pass
• pixi run test full suite including integration tests passes
• mkdocs build --strict docs build passes
• CI checks pass on this PR

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