Multi shard consolidation - #862

Additive feature for consolidating many SEG-Y shards (e.g. a shot survey split
across files) into one MDIO store, cloud-to-cloud, without changing the default
single-file segy_to_mdio behavior.
- allocate_mdio_grid / append_segy_shard: two-phase region-write workflow that
 builds an empty global grid, then writes each shard into its region in place.
- plan_consolidation (+ ConsolidationPlan): deterministic planner computing per
 shard chunk ownership, per-chunk fill vs merge (read-modify-write) mode, the
 shard conflict graph, and concurrency waves. Orchestration is left to callers.
- Optional read-modify-write in trace_worker / blocked_io.to_zarr via a
 merge_chunks set, so shards sharing a boundary chunk don't clobber each other.
 Defaults to the original fast pure-write path when not provided.
- Export the new API from mdio, mdio.converters.

A shared template's "required" set should describe the data category, not one
contributor's acquisition rig. `gun` (StreamerShotGathers3D) and the new
elevation coords (ShotReceiverLineGathers3D) are now optional coordinates:
populated when the source carries them, silently omitted otherwise.
- base: add _optional_coord_names + optional_coordinate_names property
- validation: subtract optional coords from required-field check; add
 prune_absent_optional_coordinates to drop them from the resolved schema
 when the segy_spec doesn't carry them
- pipeline + consolidate: prune optional coords after schema resolution
- shot-receiver-line: add optional source/receiver elevation for true 3D
 land/areal geometry
Co-authored-by: Cursor <cursoragent@cursor.com>

Introduce an optional shard grid so many chunks pack into one storage object
(far fewer objects / S3 GETs) while keeping the small chunk as the partial-read
unit. Adds shard_grid to VariableMetadata and shard_shape to ResolvedSchema,
a full_shard_shape property on the template base, resolver pass-through, and
shard resolution/validation (whole multiple of the chunk per dimension) in the
dataset factory. The xarray builder emits the sharding_indexed codec (v3 only)
and aligns Dask lazy blocks to shard boundaries so to_zarr's safe-chunks guard
passes; blocked_io writes whole shards to avoid read-modify-write races.
Headers are intentionally left unsharded: their structured (void) dtype trips
Zarr's sharding partial-encode path, and they are tiny relative to the sample
cube. Sharding applies to the data variable.
Co-authored-by: Cursor <cursoragent@cursor.com>