Fix stable C# generic outline signatures

[Widthdom]

Summary

• derive stable, readable display signatures for generic C# callables while preserving canonical symbol identity, exact navigation, and old-index compatibility
• normalize generic arity and type-parameter references across overloads, extension/local/explicit-interface methods, nested tuples, nullable/array types, attributes, defaults, comments, escaped identifiers, and Unicode escapes
• document the display/fallback contract in the bilingual user and developer guides and add a bilingual changelog fragment

Root cause

The outline display builder treated callable parsing as an ordinary parameter-list lookup. Generic declarations could not reliably identify the method type-parameter list or lexically split complex C# parameters, so they fell back to line-based names such as Name@line.

Impact

Generic overloads now receive distinct, line-stable display names such as Convert<T>(T) and Convert<T1, T2>(T1, ref T2). Canonical paths, stored symbol identity, and exact-query aliases are unchanged. Incomplete legacy signatures retain the existing Name@line fallback.

Fixes #4916

Documentation and changelog

• updated the English and Japanese sections of README.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md
• added the bilingual fragment changelog.d/unreleased/4916.fixed.md
• did not edit CHANGELOG.md

Validation

• focused Issue Emit stable signatures for generic C# method display names #4916 regression test: net8.0 and net9.0 passed
• all DbReaderTests after the final review fixes: net8.0 682 passed; net9.0 681 passed, 1 skipped
• full net9.0 Release suite: 10,612 passed, 420 skipped
• full net8.0 Release suite: 11,083 passed, 7 skipped; three shared-load timing tests failed in-suite and each passed on isolated retry
• dotnet build CodeIndex.sln -c Release --no-restore: 0 warnings, 0 errors
• dotnet format CodeIndex.sln --no-restore --verify-no-changes
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check
• post-commit local index refresh and status --check --json: fresh, complete graph/reference/fold state, no failed checks

Adversarial review

Two Codex adversarial-review rounds were completed, the workflow maximum. Round-one findings covering placeholder collisions, lexical Unicode normalization, delimiter splitting, and Unicode identifiers were addressed. Round-two findings covering nested interpolated defaults, attribute comparison expressions, and comment trivia around generic delimiters were also addressed and regression-tested.

Follow-up candidates

None identified within the scope of Issue #4916.