feat[next]: declare connectivities as classes; FieldOffset derived from them

[egparedes]

Stacked on #2907 (PR 3). Part 4 of the connectivities as types stack (ADR 0028, ADR 0029).

What

The tree switches from FieldOffset + separately declared local dimensions to connectivity declarations:

# before
class V2EDim(gtx.DimensionIndex, kind=gtx.DimensionKind.LOCAL): ...
V2E = gtx.FieldOffset(V2EDim.tag, source=Edge, target=(Vertex, V2EDim))

# after
class V2E(gtx.NeighborConnectivity[Vertex, Edge]):
    class Local(gtx.LocalDimensionIndex): ...
V2EDim = V2E.Local  # only where existing code keeps using the old name

• Fixtures and tests: toy_connectivity.py (V2E, E2V, C2E, V2V), cases_utils.py (imports them and declares C2V), fvm_nabla_setup.py, and the unit tests that declared their own offsets. The provider keys X.value become X.Local.tag. PR 2 already wrote the ITIR-level strings symbolically as XDim.tag, so they stay valid with no edits.
• Docs: the Quickstart guide (declares E2C/C2E, uses E2C.Local), workshop helpers.py, and slides_2.
• Local dimensions must subclass LocalDimensionIndex. A DimensionIndex subclass declared with kind=DimensionKind.LOCAL is now a TypeError that names the replacement, so a local dimension always has an owner slot. ConstListDim moves under LocalDimensionIndex.
• The two-target FieldOffset is deprecated, with a DeprecationWarning pointing at NeighborConnectivity. The offsets derived from a declaration don't warn. Cartesian FieldOffsets (used only by as_offset) stay until PR 6.
• Iterator-level embedded shift/neighbors and iterator tracing accept a connectivity class.

FieldOffset stays in a few tests that exercise the legacy path until PR 6 removes it.

Verification

unit 2256 passed, integration + regression 3803 passed / 0 failed (-n 2), notebooks (slides, exercise solutions, Quickstart, advanced, examples) all pass, src doctests pass, pre-commit --all-files clean.

Review

An independent adversarial review approved this PR. Its one design point was connectivities that share a local dimension, like ICON4Py's C2CE. They are now supported by the last commit of #2907, and PR 5 adds the backend support.

[egparedes]

Absorbed into #2910: the stack was shortened from 7 PRs to 4. Declaring the tree's connectivities as classes now lands together with class-keyed offset providers and the removal of FieldOffset, as a single migration of the tree; the intermediate FieldOffset-derived-from-a-declaration shim and its DeprecationWarning are not carried over.