feat: add regression coverage score metric

[MaxPolak97]

What does this PR do?

Adds the Regression Coverage Score (RCS) metric to openstef-beam for evaluating probabilistic forecast interval calibration.

RCS measures the fraction of observed values that fall within a predicted quantile interval, such as P10-P90. The metric is implemented as rcs(...) in the deterministic metrics module, exported through openstef_beam.metrics, and exposed through RCSProvider for evaluation pipelines. The provider follows the same symmetric quantile-pair logic as RIQDProvider.

Closes #1064

Type of change

• Bug fix
• New feature
• Breaking change (see checklist below)
• Documentation
• Refactor / chore / CI

Breaking changes checklist

• Public API, config schema, or serialized/pickled objects changed in a way that affects existing users
• If yes: see the breaking changes guide for the pickle/_migrate_state migration pattern

Migration path for existing users (e.g. "old pickled XScaler objects auto-migrate on load", "users must now pass X explicitly"):

AI disclosure

• No AI assistance was used (beyond grammar/spelling)
• AI assistance was used — tool(s):
  • I have reviewed, understand, and can explain all AI-generated code in this PR
  • This is disclosed in a commit message (e.g. Assisted-by: <tool name>)

Checklist

• poe all --check passes locally
• Tests added/updated for the change
• Documentation updated (docstrings, user guide, examples) if needed
• Commits are signed off per our DCO (git commit -s)
• PR title follows Conventional Commits (e.g. feat: ..., fix: ..., feat!: ... for breaking changes)

[egordm]

Code looks great now! Thanks for your contribution.
Merging.

[lschilders]

Nice improvement, LGTM!