v0.1.4: --prerelease CLI flag on update + update check

[StuartMeeks]

Summary

• Adds --prerelease to update and update check so downstream apps (e.g. pl-app) can opt into prerelease tags for one command invocation without touching the DI-registered SelfUpdaterOptions.IncludePrereleases default. Absence of the flag preserves today's behaviour exactly.
• Threads a bool? includePrereleasesOverride through the source-query chain (IUpdateSource.GetLatestAsync → IUpdateChecker.CheckAsync → ISelfUpdater.GetLatestReleaseAsync) as default interface methods, so existing external IUpdateSource implementers compile unchanged. null defers to the source's captured default; true/false force inclusion or exclusion.
• The update-check cache now keys on (channel, includePrereleases) so a --prerelease answer doesn't pollute the next default update check (and vice versa). v0.1.3 cache files read as non-prerelease — matches their actual provenance.

Why DIM over modifying interface signatures inline?

v0.1.3 set a precedent for inline parameter additions (onConflict on ISelfUpdater.InstallAsync), but the README bills IUpdateSource as the primary extension point. Breaking it on a patch bump risks downstream churn that a DIM avoids. External implementers can opt into the new overload later if they want to honour the flag.

Out of scope (deliberately)

• Extending the parameterless ISelfUpdater.InstallAsync(progress, ct) for symmetry — CLI doesn't use that path (TOCTOU-safe GetLatestReleaseAsync + InstallAsync(release, ...) instead). Can be added later if a library caller needs it.

Test plan

• dotnet build — clean (TreatWarningsAsErrors=true).
• dotnet test — 171 passing (was 164; 7 new).
  • Command-level: flag-on/flag-off override plumbing for both update and update check.
  • Source-level: HttpGitHubReleaseSource hits /releases (not /releases/latest) when includePrereleasesOverride: true even if constructed with includePrereleases: false.
  • Checker-level: cache key isolation between default and prerelease answers.
• dotnet build -c Release — produces NextIteration.SpectreConsole.SelfUpdate.0.1.4.nupkg + .snupkg in artifacts/packages/.
• Maintainer to verify against a downstream consumer (e.g. pl-app update check --prerelease against a repo with an RC tag) before cutting the release.
• Maintainer to publish to NuGet.

🤖 Generated with Claude Code