symbols/definition do not accept a --visibility filter even though visibility is stored in the DB

[Widthdom]

Summary

cdidx symbols and cdidx definition accept --kind (function/class/etc.) but do not accept --visibility (public/internal/protected/private). Audit workflows like "list public API surface" or "find internal helpers that should be private" have to re-derive visibility from cdidx unused output (which prints [private]/[internal]/[public] markers).

The DB already stores s.visibility (e.g. used in src/CodeIndex/Database/DbReader.CSharpResolution.cs:923), so a --visibility filter is a one-line query-builder change.

Use cases

• "List the C# public surface of this assembly": cdidx symbols --lang csharp --visibility public
• "Find internal helpers that should be made private": cdidx symbols --lang csharp --visibility internal --max-references 3
• "Show all private test helpers": cdidx symbols --lang csharp --visibility private --path tests/**

Proposed direction

Add a --visibility flag to symbols, definition, unused, and hotspots, with values public | protected | internal | private (multi-value comma-separated). Add a corresponding --exclude-visibility for negation. Reuse the same parsing as --kind.

Repro env

• Branch: main @ 2ee912d (release v1.21.0)

[Widthdom]

Work started on branch fix-issue1412.