Add per-domain input union with Grid variant and match dispatch

[chuckwondo]

Summary

First of three stacked PRs splitting #18's Story-3 modeler work (part 2a).
Introduces the per-domain input union with only the Grid variant — a
behavior-preserving refactor plus the dispatch foundation that #23
(Point/PointSeries) and #24 (CoverageCollection) build on.

What changed

• input.py: replace the single CoverageInput dataclass with
  _CoverageInputBase + GridInput; keep CoverageInput as a one-member
  type alias. Validation splits into a shared base __post_init__ plus a
  per-variant _validate_shape hook (GridInput requires 3-D
  (bands, height, width)).
• imagedata_to_coverage_input() now returns GridInput and drops its
  geometry/timestamps parameters — on the Grid path they only ever
  reached NotImplementedError.
• modeler.to_coverage: match/assert_never dispatch replacing the
  Grid-only guards. assert_never is version-guarded (stdlib on 3.11+,
  typing_extensions on 3.10), so adding a variant in Modeler part 2b: Point and PointSeries domains #23 without a case
  becomes a mypy error. typing-extensions is declared as a
  python_version < "3.11" conditional dependency.
• Tests migrated to GridInput; the obsolete 2-D / geometry / timestamps
  and modeler-guard tests are removed (they return as Point-variant tests in
  Modeler part 2b: Point and PointSeries domains #23).
• Docs: fold the decisions into docs/04-modeler-converter-design.md
  (Sections 4/4.1/7 + new 7.5, status callouts on the superseded 3/5.1), and
  refresh the resolved Polygon / covjson-pydantic 0.8.0 note.

Behavior

Behavior-preserving for the Grid path — emitted CoverageJSON is unchanged.
The one new behavior: a non-3-D array is rejected at GridInput construction
(ValueError) instead of reaching the modeler.

Testing

uv run pytest (251 passed, 100% coverage), ruff check, ruff format --check, and mypy all green on Python 3.10 and 3.13.

Part of #18. Closes #22.

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