refactor[next]: MultiDimensionIndex and typed embedded positions

[egparedes]

Stacked on #2910. Part 4 of 4 of the connectivities as types stack, and the last one (ADR 0030, updated here).

What

• MultiDimensionIndex[D, *Ls] (gt4py.next.MultiDimensionIndex): a position in the product of a primary dimension and local dimensions, e.g. MultiDimensionIndex(Vertex(3), V2E.Local(1)), the second neighbor of vertex 3. It is a tuple subclass, so it indexes a field or a neighbor table directly, compares and hashes like the plain tuple, and survives pickle and copy. A TypeVarTuple cannot carry a bound, so the constructor checks the shape at runtime: one primary index followed by at least one local index, and an owned local dimension must index the neighbors of the primary index's dimension (MultiDimensionIndex(Edge(1), V2E.Local(1)) is a TypeError, since V2E.domain is Vertex).
• Typed positions in the iterator-level embedded execution. Positions, named indices and domains are keyed by dimension classes instead of tag strings. SparseTag is replaced by an explicit SparseAxis(dim) offset part, and the tag round trip (_dimension_to_tag) and the resolve calls on sparse lookups are gone.

Tests

• MultiDimensionIndex: indexing a table, tuple equality and hash, pickling, rejected shapes. Plus a typing test of its revealed type and unpacking.
• Embedded domains reject axes that are not dimensions; a sparse axis steps along its local dimension.
• The iterator-embedded changes are otherwise covered by the existing iterator unit and integration tests (embedded and roundtrip).

Verification

Run locally (CPU) on this head; 0 failures in every row that ran:

Check	3.12	3.13	3.14
test_next (internal, cpu, nomesh): embedded, roundtrip, gtfn	5078 passed	not run	passed (CI)
test_next (dace, cpu, nomesh)	1511 passed	not run	passed (CI)
src/gt4py/next doctests (in each session)	120 passed
test_typing_exports (mypy cases, pyright probes)	21 passed, pyright 0 errors	21 passed, pyright 0 errors	21 passed, pyright 0 errors
pre-commit run --all-files	clean

GitHub CI on the pushed head passes every check: test_next (internal and dace; nomesh and atlas) on 3.12 and 3.14, test_cartesian, test_eve, test_storage, notebooks, package, typing-exports and code quality. The local 3.13/3.14 runs were stopped in favour of CI.