Fix search ranking and guarded search bounds

[Widthdom]

Summary

• Boost ordinary search ordering for punctuation-heavy literal code phrases so exact substring hits rank ahead of punctuation-only noise.
• Bound guarded search candidate collection before pagination and return a validation error when a guarded query is too broad for the capped budget.
• Document the guarded-search broad-query error and add issue changelog fragments.

Validation

• dotnet build CodeIndex.sln -p:UseSharedCompilation=false -m:1 -nr:false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~DbSearchReaderIssueTests|FullyQualifiedName~DbReaderTests.Search_GuardFiltersReadAcrossChunkBoundaries_Issue2852" -p:UseSharedCompilation=false -m:1 -nr:false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter DbSearchReaderIssueTests
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• git diff --check
• Codex adversarial review: No blocking/actionable issues found.

Documentation / Changelog

• USER_GUIDE.md
• changelog.d/unreleased/2998.fixed.md
• changelog.d/unreleased/3082.fixed.md

Fixes

Fixes #2998
Fixes #3082

Follow-up candidates

None.