release: search, mail (inbound/SMTP/queue), dotnet-new templates, and DocFX tutorials

[tgiachi]

Summary

Release of the modules accumulated on develop since v0.4.0.

New packages

• SquidStd.Search.Abstractions / SquidStd.Search.Elasticsearch — tag entities with IIndexableEntity + [SearchIndex] (env-expanded index names), index via ISearchService, and query with a constrained LINQ IQueryable<T> translated to the Elasticsearch DSL.
• SquidStd.Mail.Abstractions / SquidStd.Mail.MailKit — IMAP/POP3 polling that publishes MailReceivedEvent, plus an SMTP IMailSender (OutgoingMailMessage, MailSentEvent/MailSendFailedEvent).
• SquidStd.Mail.Queue — outbound send queue over IMessageQueue: IMailQueue.EnqueueAsync + a background consumer, with retry/backoff/dead-letter from the messaging layer.
• SquidStd.Templates — dotnet new template pack (squidstd-host, squidstd-aspnetcore, squidstd-worker, squidstd-manager) with pack-time version injection.

Docs

• New Tutorials section in DocFX: 15 scenario guides covering all 28 packages, with code embedded from 14 compilable samples/ projects (built in CI).
• READMEs, DocFX articles and root index rows for every new package; repository-wide code formatting.

Test Plan

• dotnet build SquidStd.slnx — solution + all samples compile clean
• dotnet test SquidStd.slnx — 620/620 green
• Testcontainers integration tests pass (Elasticsearch, GreenMail IMAP/POP3/SMTP, RabbitMQ workers e2e, Redis, MinIO)
• DocFX build: 0 warnings, 0 errors (tutorials + snippet includes resolve)

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[github-advanced-security]

CodeQL found more than 20 potential problems in the proposed changes. Check the Files changed tab for more details.