Restrict Sphinx missing-reference fallback to known type aliases

[Copilot]

This narrows the docs missing-reference workaround so it only applies to the known RefinementParameter alias-resolution gap, instead of retrying every unresolved Python class reference as py:obj. The goal is to keep the alias link fix while preserving nitpicky warnings for genuine unresolved class refs.

• Scope the fallback to explicit alias targets

  • add a small allowlist for:
    • RefinementParameter
    • powderline.schema.RefinementParameter
  • only retry unresolved py:class references for those targets
  • leave all other unresolved class references untouched so Sphinx still warns normally

• Document the intended behavior

  • update the fallback comment/docstring in docs/conf.py to state that this is an intentionally narrow alias-as-data workaround, not a general missing-reference recovery path

• Keep the docs-warning cleanup aligned

  • update docs/known_issues.md to describe the restricted fallback accurately
  • add focused coverage for the fallback contract:
    • known alias targets retry as py:obj
    • non-allowlisted / wrong-role refs return None

Example of the narrowed behavior:

_TYPE_ALIAS_OBJ_FALLBACK_TARGETS = {
    "RefinementParameter",
    "powderline.schema.RefinementParameter",
}

if (
    node.get("refdomain") != "py"
    or node.get("reftype") != "class"
    or node.get("reftarget") not in _TYPE_ALIAS_OBJ_FALLBACK_TARGETS
):
    return None