Improve inspect body paging and mode metadata

[Widthdom]

Summary

• Add inspect --body-start and --body-lines for line-based definition body paging.
• Add body slice metadata and body_mode guidance to inspect JSON output, plus human body hints.
• Make byte-capped body continuation monotonic so advertised next lines do not loop.

Validation

• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~QueryCommandRunnerTests.RunInspect_BodyRangeJson_ReportsNextLineFromByteClampedContent_Issue3394|FullyQualifiedName~QueryCommandRunnerTests.RunInspect_BodyRangeJson_PagesDefinitionBody_Issue3394|FullyQualifiedName~QueryCommandRunnerTests.RunInspect_JsonWithoutBody_IncludesBodyModeHint_Issue3441|FullyQualifiedName~QueryCommandRunnerTests.RunInspect_HumanWithoutBody_PrintsBodyModeHint_Issue3441" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~RunInspect" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~ConsoleUiTests.PrintUsage_WithoutBanner_HidesAsciiArtAndEasterEggFlags" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: No blocking/actionable issues found.

Full dotnet test CodeIndex.sln -p:UseSharedCompilation=false was not rerun to completion after the focused fixes; the earlier full-suite attempt was interrupted after an expected usage-string failure was fixed.

Documentation And Changelog

• Updated USER_GUIDE.md.
• Added changelog.d/unreleased/3394.added.md.
• Added changelog.d/unreleased/3441.changed.md.

Follow-Ups

None.

Fixes #3394
Fixes #3441