Fix platform path separator handling

[Widthdom]

Summary

• Preserve literal POSIX backslashes when normalizing ignore-rule comparison paths.
• Harden project-root escape checks so Windows backslash traversal is rejected without treating POSIX backslashes as separators.
• Update CLI project path detection to use platform path syntax and Windows drive/UNC recognition.

Validation

• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~FileIndexerTests.NormalizeIgnorePath_PosixPreservesLiteralBackslash|FullyQualifiedName~IndexCommandRunnerTests.IsOutsideProjectRoot|FullyQualifiedName~ProgramRunnerTests.IsProjectPathArg" -p:UseSharedCompilation=false
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --commits a57ed0cce0f9fd2f855eca6842ea0ba1539486eb 88652f15e2447ceb6057a2f8f625a6f8f292a58c 2aa93b66fdc26ce2b2e639691343779419ba3bc8 --json
• Adversarial review: No blocking/actionable issues found.

Documentation and Changelog

• Added changelog.d/unreleased/1715.fixed.md.
• Added changelog.d/unreleased/1717.fixed.md.
• Added changelog.d/unreleased/1725.fixed.md.

Follow-up Candidates

• Existing full-index stall issues were already present as index: full scan stalls extracting SqlReferenceExtractor.cs symbols #2710 and Full index can stall on current repository after branch refresh #2717; no new issue was opened.

Fixes #1715
Fixes #1717
Fixes #1725