Add OBN Continuous Receiver Gathers (CRG) template

[andgineer]

Draft — opening early to start the discussion on the two API questions at the bottom. Code and tests are complete and green locally; happy to reshape the public surface before this is reviewed properly.

What this adds

A template for OBN continuous receiver gathers (CRG). Unlike shot-organised OBN data, CRG receivers record continuously, so there is no shot index to grid on. The natural key is (component, receiver_line, receiver), and each receiver holds a long run of time segments sharing that key.

Rather than introduce a new grid-override mechanism, this extends the existing HasDuplicates path, which already solves exactly this problem — it appends a per-tuple counter as a synthetic trace dimension. Two knobs were missing for the CRG scale, so the first commit adds them and the second adds the template.

Resulting layout: (component, receiver_line, receiver, trace, time).

Commits

1. Expose chunk size and counter dtype on the HasDuplicates path

The inserted trace dimension was hardcoded to chunk size 1 and an int16 counter. That caps a gather at ~32k duplicates and forces one chunk per trace. A full CRG campaign runs to roughly 2.6e5 segments per receiver, and chunk-per-trace gives unusably small objects.

• GridOverrides.chunksize now applies to has_duplicates as well as non_binned.
• New optional GridOverrides.trace_dtype selects the counter dtype.

Both default to the current behaviour (chunk 1, int16), so payloads that omit them are byte-identical to today. There is an explicit backwards-compatibility test for that.

The same commit generalizes _resolve_synthesize_dims to read a template's synthesize_missing_dims attribute instead of isinstance-checking Seismic3DObnReceiverGathersTemplate. The OBN template already sets that attribute, so its behaviour is unchanged; this just lets other templates opt in. ingestion/segy/validation.py keys its optional-dimension exemption off the same attribute so the two cannot drift.

2. Add OBN Continuous Receiver Gathers (CRG) template

ObnContinuousReceiverGathers3D, registered as a built-in. Grids on the three spatial dimensions and leaves the segment axis to HasDuplicates. component is declared in synthesize_missing_dims, so single-component files carrying no component header ingest against the same template with the dimension synthesized — one template covers both layouts.

Tests

tests/unit tests/integration -m "not cloud" → 560 passed.

• Template contract, coordinate-spec sync, registry inventory.
• Grid-override plumbing: inserted trace honours chunksize and trace_dtype, plus a BC test asserting the default path still yields chunk 1 / int16.
• Two integration tests ingesting mock CRG SEG-Y, one multi-component and one exercising the synthesized-component path.

Questions for maintainers

1. Is the API shape right for the two new knobs? I put trace_dtype on GridOverrides alongside the existing chunksize, since that is where non_binned already carries its chunk size and it keeps the two paths symmetric. The alternative is to carry them on the template instead, which would keep GridOverrides narrower but split the configuration for one dimension across two objects. Happy to move it.

2. Is "extend HasDuplicates" the direction you want? The alternative is a dedicated override (something like CalculateTimeSegmentIndex) that does not touch the existing path at all. Extending seemed better because the semantics are genuinely identical — a counter disambiguating repeated index tuples — and a separate override would duplicate the machinery. But it does widen a shipping public surface, so say the word if you would rather have this isolated.