feat: add load_export() reader API

[amrit110]

Summary

Closes #17.

Adds load_export(path) as the read-side counterpart to export_batch(), closing the write/read asymmetry where users had to manually parse NPZ array keys and JSON manifests to recover their embeddings.

The loop is now symmetric:

# Write
export_batch(
    spatials=[PointBuffer(121.5, 31.2, 2048)],
    temporal=TemporalSpec.range("2022-06-01", "2022-09-01"),
    models=["remoteclip"],
    target=ExportTarget.combined("exports/run"),
)

# Read
result = load_export("exports/run.npz")
emb = result.embedding("remoteclip")   # shape (n_items, dim)

Changes

• src/rs_embed/load.py — new module implementing load_export, ExportResult, and ModelResult
  • Auto-detects combined (single .npz/.nc file) vs per-item (directory) layout
  • NaN-fills failed spatial points in partial runs; surfaces status="partial"
  • result.embedding(model) typed shortcut; ok_models / failed_models convenience properties
  • NetCDF support via optional xarray import
• src/rs_embed/__init__.py — exports load_export, ExportResult, ModelResult at top level
• src/rs_embed/api.py — removes legacy private re-exports (_default_provider_backend_for_api etc.) that were kept with # noqa: F401
• tests/test_load_export.py — 45 unit tests covering both layouts, path routing, partial failures, NaN fill, two-model runs, and error cases
• tests/test_backend_resolution.py — updated to import from canonical location (tools/normalization) now that the api.py re-exports are gone
• docs/api_load.md — new docs page with full API reference, data structure docs, and usage examples
• docs/api.md, docs/api_export.md, mkdocs.yml — nav and cross-links updated
• CHANGELOG.md — [Unreleased] entry added
• .pre-commit-config.yaml — fixes hook id ruff → ruff-check (the non-legacy name)

Test plan

• python -m pytest tests/test_load_export.py -v — all 45 tests pass
• python -m pytest — full suite (570 tests) passes
• pre-commit run --all-files — prettier, ruff-check, ruff-format all pass
• mkdocs build --strict — docs build clean (pre-existing extending.md error unrelated to this PR)