Add from_polytope_reforecast() for hdate support across all encoders

[awarde96]

Description

Adds from_polytope_reforecast() to the base Encoder class. Under the hood, from_polytope() gets a date_key parameter (defaults to "date" so nothing changes for existing callers). from_polytope_reforecast() just calls from_polytope(result, date_key="hdate").

This means all 10 encoders now support reforecast data where hdate is the time axis instead of date. Each hdate value produces its own coverage — no merging across hdates.

Scope decisions

• Reforecast only, not reanalysis: We originally considered merging multiple hdate values into a single coverage with an expanded t-axis (for reanalysis use cases like CEMS-floods). That's descoped and each hdate is a separate coverage, which is the correct behavior for reforecasts. The more complex merging can be revisited later if needed.
• "Forecast date" metadata: When using hdate, the hdate value ends up as mars_metadata["Forecast date"]. For reforecasts this probably makes sense (hdate is the forecast init time). For pure reanalysis this naming is arguably wrong but we're not targeting that use case here.
• Decoders not updated: The existing decoders should still work since the output is standard CoverageJSON, just with different metadata values. But there are no decoder-specific hdate tests. Decoder validation should be a follow-up.

Tests

The test_encoder_*_from_polytope.py files are regression tests, not realistic e2e examples. The tree fixtures exercise the encoder code paths and verify the CoverageJSON structure but use small/simplified inputs (e.g. 2 spatial points for a bounding box, multiple steps for Position). They should be sufficient for catching regressions introduced in this PR but more realistic integration tests with actual polytope output would be a good follow-up.

Also fixed

Circle.py had return self.covjsonå (non-ASCII typo) instead of return self.covjson in from_xarray. Pre-existing on main, fixed here since we were touching the file.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[awarde96]

@copilot

[Copilot]

@copilot

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[awarde96]

@copilot please review this PR

[Copilot]

@copilot please review this PR

Here's my review of the PR:

Overall: the approach is clean and all 159 tests pass.

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What the PR does well

• Backwards-compatible by design — date_key defaults to "date" everywhere so no existing caller is affected.
• Single change point — walk_tree gets the date_key parameter and propagates it recursively; all 10 subclasses just forward it through.
• from_polytope_reforecast in the base class — inheriting it for free in every encoder is the right pattern. No code duplication.
• Typo fix (covjsonå) is good to have bundled in.
• Test coverage — regression tests across all 10 encoders for both normal forecasts and reforecast paths, including multi-hdate/multi-step cases.

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Issues / things to be aware of

1. walk_tree_step and walk_tree_month still hardcode "date".
   from_polytope_reforecast only calls from_polytope which routes through walk_tree, so this doesn't cause a regression right now. But if someone later adds a from_polytope_step_reforecast or from_polytope_month_reforecast, they'll hit the same problem. Worth a follow-up note/issue.

2. mars_metadata["Forecast date"] is always written, even when date_key="hdate".
   The PR description acknowledges this. For reforecasts it's reasonable (hdate is the forecast init date). For reanalysis it's misleading. Since pure reanalysis is out of scope for this PR, it's acceptable as-is but worth tracking.

3. Abstract method signature mismatch.
   from_polytope is declared @abstractmethod with date_key: str = "date" in the base class. All 10 subclasses override it with the same signature. That's correct. However, from_polytope_reforecast is not abstract — it's concrete in the base class. That's intentional and the right design, just worth noting for anyone adding a new encoder subclass: they don't need to override from_polytope_reforecast.

4. No docstring update in the Encoder class docstring — the methods listed in the Attributes/Methods section in __init__ still only mention from_polytope(result) (the old signature), not from_polytope_reforecast. Minor nit.

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Summary

The implementation is sound, the scope decisions are well-reasoned, and the tests cover the important cases. The open items above are either pre-existing or acknowledged in the PR description. The PR is ready to merge with no blocking issues.