Support prerelease versions in the release and CI workflows

[Vulthil]

Summary

Groundwork for a preview/net11 release train: a long-lived branch whose version.json carries a -preview.{height} prerelease tag and whose packages additionally target net11.0, released through the existing Release workflow. These are the main-side changes; they are inert for stable releases.

• release.yml: gh release create passes --prerelease for versions with a prerelease tag, so a preview never becomes the repo's "Latest" release. The docs, mark-api-shipped, and propagate-baseline-to-main jobs skip prerelease versions: the site default must never point at a preview, the PublicAPI freeze + PackageValidationBaselineVersion bump are stable-release rituals, and propagate_baseline.py only ever raises — a 1.3.0-preview.N out-ranks the latest stable and would poison main into validating against a non-stable baseline. Also corrects the PublicRelease=true comment: it drops the non-public -g<sha> suffix; a prerelease tag from version.json survives it.
• ci.yml: preview/** branches get CI (push + PR), and the pack/upload/publish-preview conditions now include refs/heads/preview/, so merges to a preview train branch publish prerelease packages to GitHub Packages the same way main merges do. Coverage badge/artifact side effects stay main-only.
• MessageConfiguration.DefaultUrnFromFullName: string.LastIndexOf(char) → AsSpan().LastIndexOf(char). .NET 11 adds a LastIndexOf(char, StringComparison) overload, so CA1307 (via AnalysisMode=All + TreatWarningsAsErrors) fails the net11.0 compile; the span overload is ordinal by construction on every TFM and keeps the preview branch source-identical to main. A string-overload fix (LastIndexOf(".", StringComparison.Ordinal)) would trip CA1866 instead.

Verification

• Both workflows parse as YAML; conditions reviewed against stable-release dispatches from main and servicing branches (no behavior change: version has no -, so all guards stay true, and preview/** filters don't affect existing refs).
• Preview-train changes (net11.0 TFM in src, SDK 11.0.100-preview.6, 1.3.0-preview.{height}) applied locally on top of this branch: full solution builds with 0 warnings/0 errors, dotnet pack passes package validation against the 1.2.0 baseline including the new net10.0↔net11.0 ApiCompat pair with no new suppressions, and all 24 nupkgs contain lib/net11.0 with correct per-TFM dependency groups.
• This branch alone on SDK 10.0.302: full solution builds with 0 warnings/0 errors; Vulthil.Messaging tests pass 116/116.

Backport to v1.0: no