feat[next]: backends support connectivities sharing a local dimension

[egparedes]

Stacked on #2908 (PR 4). Part 5 of the connectivities as types stack (ADR 0029).

What

#2907 lets a connectivity share another connectivity's local dimension, e.g. ICON4Py's flattened C2CE indexing the same neighbor axis as C2E. The sharer is named in the IR by its own tag (offset_tag), because the local dimension's tag already names the owner's table. That brings back an offset tag that differs from its local dimension's tag, which PR 1's regression matrix had marked xfail on several backends. This PR makes all of them work.

• common.connectivity_key_over(provider, local_dim) finds the key of a bound table over a local dimension. That is the table keyed by the local dimension's tag (its owner's) if bound, and otherwise any table whose neighbor dimension it is (a sharer's).
• It replaces the lookups by the local dimension's tag in:
  • embedded reductions (nd_array_field)
  • unroll_reduce
  • gtfn sparse arguments
  • iterator-embedded sparse lists, sparse positions and list materialization
  • DaCe make_field, reductions with skip values, and map_list
• _List (iterator embedded) records the connectivity it was built with. Writing a list back goes along that connectivity's local dimension, not a dimension named after the offset.
• DaCe: a connectivity array's local dimension is sized from that connectivity's own table (sdfg_args.local_dimension_size). This fixes the DaCe shift through an offset whose tag differs from its local dimension.

So a sharing connectivity works on every backend, whether it is bound alone or together with its owner.

Tests

• test_neighbor_connectivity.py: shift and reduction through a shared local dimension, mixed with the owner's neighbors, and with the owner not bound at all. The owner and the sharer are bound to different tables, so using the wrong one fails.
• PR 1's markers uses_offset_tag_differing_from_local_dim and …_in_reduction are removed together with their skip-list entries: every cell they marked passes now.

Verification

unit + integration + regression: 6101 passed, 0 failed (-n 2); pre-commit --all-files clean.

Deviation from the plan

The plan's PR 5 was "backends resolve connectivities through Local.owner". PR 2 made one string name the offset, the local dimension and the provider key, and PR 6 keeps the internal provider keyed by tags, so going through owner would change nothing. What the backends actually lacked was a way to find a table over a local dimension when it isn't keyed by that dimension's tag.

[egparedes]

Absorbed into #2907 (stack #2917): the backend support for connectivities sharing a local dimension now lands together with the declarations.