Index.from_chunks(): manifest-built indices for the parquet-primary sidecar store

[espg]

Closes #1. Implemented exactly per the plan posted there
(#1 (comment)).

What

• Index.from_chunks(source, datasets) (staticmethod): construct a
  decode-capable index from an externally-supplied chunk manifest — no
  granule file access, no stored bincode. Plain python/numpy inputs only;
  parquet decoding stays caller-side.
• Index.filters(dataset) → {"gzip": int|None, "shuffle": bool, "byte_order": "little"|"big"|"unknown"} — the missing extractor-side
  getter; datasets()/shape()/chunk_shape()/dtype()/filters()/chunks() now
  produce everything from_chunks consumes.
• README: usage, the extractor metadata contract, the filter_mask
  limitation.

Construction route (hybrid, as planned)

The per-dataset payload is built entirely with hidefix 0.12 public API
(Chunk::new, Dataset::new, DatasetD::D1..D9, Datatype, Order).
The two non-constructible shells (Index, GroupIndex — private fields)
go through in-memory serde shims with bincode-identical layout, serialized
and immediately deserialized through the same code path load() uses
(shared holder_from_bytes helper). Bincode never touches disk.

Validation / errors

ValueError with dataset name + offending key on: missing keys, rank
mismatches, chunk-table length mismatches, wrong chunk count vs
ceil(shape/chunk_shape), non-chunk-aligned or out-of-bounds offsets,
duplicate offsets, unsupported dtype, gzip given as bool, and any nonzero
filter_mask (hidefix's chunk model is (addr, size, offset) — per-chunk
filter masks are unrepresentable; ATL03 masks are all zero per
englacial/zagg PR #159). Chunk rows are sorted internally.

Tests

45 total (was 37), all green both tiers:

• hermetic: extract-via-getters → from_chunks → byte-identical
  read/read_plan/read_from_buffers vs the directly-built index
  (incl. empty + boundary-crossing ranges), filters() values vs h5py,
  save/load coherence of a from_chunks index, unsorted rows, plain-list
  inputs, no-file-access construction (phantom source path), and 15+ error
  paths.
• local (NEON ATL03 granule): extract → from_chunks → byte-identical direct
  and buffer-fed reads.

clippy -D warnings, fmt, and both CI workflows green.