[pournasserian]

Closes #2706.

What

• SDK FluentCMS.Seeding: ISeeder, SeedContext (services, site, locale, the host clock, InLocale) and ISeedRunner.
• Core: cms.Seeding.Add<T>(), RequireLocale(...), RunOnStart and ClaimTimeout. Seeders run in StartAsync after the migrations, on the default site, as the system principal with the Seed origin. With no seeder added nothing is added to start-up. Only seeders added with Add run, at boot and through ISeedRunner.
• What was applied (seeder Name and Version) and a claim are kept in the instance settings area seeding (no system table). A run claims the seeder with a write that names the version it read; an instance that loses skips and boots normally, and a claim older than 15 minutes can be taken over.
• SiteManager.EnsureLocaleAsync adds a locale to the default site if missing. Site management arrives in M2, and the sample needs fr.
• samples/Blog: Article, Author, Tag, Seo, HeroBlock, QuoteBlock, GalleryBlock, typed BlogSettings ([Settings("blog")]), BlogSeeder and cms.AddBlogSample().
• Seed set: 2 authors, 3 tags, 5 articles (published in en and fr, published, draft, trashed with DeleteAsync, scheduled last with ScheduleAsync 30 days after the host clock) and the site's blog settings. Everything goes through the public services.
• Spec updated in 4.16 and 6.3.

Decisions

• Entries are found by slug (authors and tags too, Author gained a unique slug), each step converges (publish if draft, add fr if missing, trash if not trashed, schedule if still a draft), a unique conflict counts as already there, and purged or renamed content does not come back because an applied seeder is skipped.
• SiteSettings is typed settings, not the singleton from 4.2, as the notes on the issue say.
• Media fields are left out of the sample until the media plugin registers the media field type. GalleryBlock has a caption and a column count for now.
• samples/Blog references Sdk and Core, to offer AddBlogSample. No system table or migration change.

Tests

BlogSampleTests and BlogSeedingTests in FluentCMS.Core.IntegrationTests (the second covers concurrent runs on a file database, purge, rename and settings change then another boot, a crash halfway then a rerun, a version bump, claim timeout, failure release) (one seeded host for the class, about 0.5 s): types register, read-back of authors, tags, articles, relations, blocks and component, the fr translation, the trashed entry only in the trash, the scheduled entry a draft with SchedulePublishAt at clock plus delay, Seed origin in the versions, settings stored, a second run creates no rows, and a host without the sample has no seed service.

dotnet build -c Release -warnaserror and the full dotnet test -c Release (2779 tests) pass locally. Markdown lint is clean. SQLite only; PostgreSQL and SQL Server follow #2711.

No new packages.