Cap detailed diff row field output

[Widthdom]

Summary

• Cap oversized encoded field samples in cdidx diff --detailed rows and include original length plus a stable SHA-256 fingerprint.
• Compare full row sort values directly so truncation does not hide drift and equal rows do not need display encoding.
• Add regression coverage for large symbol signatures and large chunk content with shared display prefixes.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~DiffCommandRunnerTests
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Documentation and changelog

• Added changelog.d/unreleased/3163.fixed.md.
• No docs update: the detailed encoded row string format is not documented as a public contract outside changelog coverage.

Review

• Adversarial review: No blocking/actionable issues found.
• codex exec review --base origin/main was attempted but could not complete in this environment because nested shell calls failed repeatedly; the final origin/main..HEAD diff was reviewed manually under .codex/workflows/adversarial-review.md.

Follow-up candidates

• None.

Fixes #3163