feat: provide the backend collection loops in the loader

[henryiii]

🤖 AI text below 🤖

Prompted by scikit-build/scikit-build-core#1433: the first real consumer of the dynamic_wheel hook had to write the collection loop itself, including validation rules our docs only stated in prose. The reference implementation should provide these loops so backends don't each re-derive spec rules.

Changes

• loader.py: two new public functions, replacing the copy-paste snippets on the backend-authors page:
  • get_requires_for_dynamic_metadata(entries) -> list[str] — extra build requirements, collected in entry order, for the backend's PEP 517 get_requires_for_build_* hooks.
  • dynamic_wheel_fields(entries) -> set[str] — the METADATA 2.2 pass, now enforcing the rules the old doc snippet lacked: reported field names are validated against ALL_FIELDS, version: True raises, and unmentioned fields default to not dynamic.
• Semantics change vs. the old snippet: same-field reports from multiple providers combine with OR (any True wins) rather than last-wins dict.update. Contributions to a field merge, so one dynamic part makes the merged value dynamic; PEP 643 permits over-marking but not under-marking. (Issue to align scikit-build-core to follow.)
• Docs: backend_authors.md calls the real functions; the full loops (with validation) move to backend_authors_reimplement.md as the copyable reference; plugin_authors.md notes dynamic_wheel may run on a fresh instance, so it cannot rely on state stashed by dynamic_metadata/build_state (it only receives settings by design).
• Tests (written first, confirmed failing without the fix): collection with hook-less providers skipped, the OR case, unknown-field rejection, and the version rule; these replace the old test that hand-rolled the loop.

The new functions appear in the API reference automatically via the existing automodule directive.