Choose a canonical representative for partial declaration families

[Widthdom]

Summary

--group-partials can choose an alphabetically early split file instead of the primary declaration, and goto offers no documented canonical-family choice.

Audit IDs: Q-20. Reproduced with locally built cdidx v1.40.3 at 404db09489c305f17d94f37026e00564d3ac527a.

Existing issue relationship

No exact existing issue was found.

Expected behavior and PR scope

Define a deterministic canonical representative and return selection reason/family members. Keep this to partial-family grouping/ranking, not constructor navigation.

Implementation guide for Codex 5.6 sol xhigh

• Specify ordered signals such as explicit primary/body declaration, non-generated source, attributes/base list/docs, path stability, and source position.
• Use canonical symbol/family IDs before path/name tie-breaks and keep the result stable across index order.
• Expose representative reason and optional family members in structured output.
• Cover partial classes/records/methods across files, generated/designer files, equal candidates, renames, and old DB metadata.
• Add focused regression/contract tests as appropriate, run the full net8.0 and net9.0 suite, and add the required bilingual changelog.d/unreleased/ fragment.

Acceptance criteria

• The representative follows the documented semantic rule, not arbitrary alphabetical order.
• Selection is deterministic across rebuilds.
• Clients can intentionally navigate the representative or full family.