Reject control characters in database checkpoint names

[Widthdom]

Summary

• Reject all C0 control characters in database checkpoint names on every supported platform.
• Apply the canonical validation to checkpoint creation, restore, and deletion before checkpoint artifacts can be created.
• Preserve accepted checkpoint-name round trips and document the contract in English and Japanese.
• Add the bilingual security changelog fragment changelog.d/unreleased/5082.security.md.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-build
  • net8.0: 11,433 passed, 7 skipped, 0 failed
  • net9.0: 10,961 passed, 418 skipped, 0 failed
• Focused checkpoint and help tests on net8.0/net9.0
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• Root index and workspace manifest health checks
• Codex adversarial review: No blocking/actionable issues found.

Fixes #5082