fix: defer siac._rust and pyhdf imports to avoid import-time failures

[Copilot]

Several optional/compiled dependencies were imported at module level, causing cascading import failures when pyhdf wasn't installed or the Rust extension (siac._rust) hadn't been compiled — breaking 22 test collection sites and masking ~168 tests.

Changes

• earthdata_common.py — Move from pyhdf.SD import SD, SDC inside read_hdf4_dataset(). HDF4 reading is an optional, infrequent path; it should not gate the entire package import.

• brdf/kernels.py, brdf_whittaker.py, rt/emulator/two_nn.py, solver/multigrid.py — Defer all from siac._rust import … calls to the point of first use (inside methods/functions). The Rust extension is a performance accelerator; classes that wrap it can now be instantiated and passed around without requiring a compiled extension, failing only when the accelerated code path is actually invoked.

  # Before — fails at import time if siac._rust not built
  from siac._rust import RossThickLiSparse as _RustKernels
  
  # After — fails only when BRDFKernels.compute() is called
  if self._rust_kernels is None:
      from siac._rust import RossThickLiSparse as _RustKernels
      self._rust_kernels = _RustKernels(self.hb, self.br)

• satellite/sentinel2.py — Sentinel2Preprocessor.sensor_config now catches failures from load_sensor_config_from_srf (network unavailable, no cache) and falls back to the built-in nominal SENTINEL2A/B/C_CONFIG. Adds missing imports for SENTINEL2B_CONFIG and SENTINEL2C_CONFIG to support all three platforms.

Impact

	Before	After
Tests passing	412	580
Collection errors	22	1 (test_solver.py directly imports siac._rust)
Tests failing	11	22 (all require siac._rust at runtime — correct)

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Original prompt

Review the current codebase in detail and provide suggestions and recommendations and implement fix's but make sure tests are passed

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