Normalize C# field initializer symbols

[Widthdom]

Summary

• Emit ordinary C# fields, including collection-expression and target-typed initializers, as field symbols while preserving real properties and existing symbol metadata.
• Replace oversized top-level initializer bodies with deterministic … markers while retaining modifiers, types, declarators, and comment-safe declaration structure.
• Bump the C# extractor contract and carry field identity through persisted receiver resolution, search declaration metadata, reference lookup, and LSP document/completion projections.

Root cause

The ordinary-field matcher reused an internal property kind, and the previous size bound truncated a flattened initializer prefix. Downstream receiver, search, and LSP paths therefore continued to assume those members were properties even after public normalization.

Validation

• dotnet build CodeIndex.sln -c Release --no-restore — passed with 0 warnings and 0 errors.
• Focused Issue Normalize C# field symbols for collection expressions and large initializers #4865 regression tests — 5 passed on net8.0 and 5 passed on net9.0.
• Full net9.0 Release suite — 10,268 passed, 420 skipped, 0 failed.
• Full net8.0 Release suite — 10,741 passed, 7 skipped, 1 unrelated watcher timing failure covered by Stabilize net8 subproject watcher reconciliation for ancestor .gitignore changes #4955; the same test passed in an isolated Debug retry and reproduced in isolated Release retries.
• dotnet format CodeIndex.sln --no-restore --verify-no-changes — passed.
• dotnet run --project tools/CodeIndex.Changelog -- check — validated all 36 fragments.
• git diff --check and cdidx status --check --json — passed; the local index matches the committed workspace.
• Two Codex adversarial-review rounds completed; all actionable findings from both rounds were addressed.

Documentation and changelog

• Updated the bilingual field-symbol contract and test guidance in DEVELOPER_GUIDE.md and TESTING_GUIDE.md.
• Added bilingual fragment changelog.d/unreleased/4865.fixed.md.

Follow-up candidates

• Stabilize net8 subproject watcher reconciliation for ancestor .gitignore changes #4955 tracks the unrelated net8 ancestor .gitignore watcher reconciliation timeout observed during Release validation.

Fixes #4865