Restore cross-file partial-class usage detection in unused analysis

[Widthdom]

Summary

unused again marks private fields as actionable-unused when their uses live in sibling files of the same partial class. Current examples include endLineExplicit, outputFormatExplicit, and _hasIssueMetadataColumns.

Audit ID: R-01. Reproduced with v1.40.3 at 404db09489c305f17d94f37026e00564d3ac527a.

Existing issue relationship

This is a direct regression of closed #3673 (“Fix unused analysis for partial-class handlers and field-like symbols”). The current reproduction must not be discarded merely because that issue was completed.

Expected behavior

Unused analysis must aggregate references across the logical partial type before declaring a field unused.

PR scope

Keep this to cross-file partial-family reachability and the three representative fields as one root cause. Do not create per-field fixes or suppress by name.

Implementation guide for Codex 5.6 sol xhigh

• Add a two/three-file partial-class fixture with used and truly unused private fields, including nested partial types.
• Resolve logical partial-family membership before usage classification; do not rely on same-file ranges or folded names alone.
• Preserve conservative behavior for reflection/config suspects and for unrelated same-named types.
• Verify compact, JSON, by-bucket, and actionable filters against the same fixture.
• Add regression tests, run full net8/net9, and add a bilingual changelog fragment referencing Fix unused analysis for partial-class handlers and field-like symbols #3673.

Acceptance criteria

• The three current fields are not reported unused.
• A genuinely unreferenced private field in the same partial family remains actionable.
• Unrelated same-named partial types do not share references.

[Widthdom]

Work started on branch fix-issue4834.