Public contract root for the mailglass_inbound sibling package.

Phase 40 keeps the stable surface narrow and explicit:

• MailglassInbound.InboundMessage is the canonical normalized inbound value object.
• MailglassInbound.Router is the adopter-owned routing DSL.
• MailglassInbound.Mailbox is the mailbox callback contract.
• MailglassInbound.Ingress.Plug is the first-party Postmark ingress entrypoint.
• MailglassInbound.Ingress.CachingBodyReader is the package-local raw-body capture helper.
• version/0 returns the package version string at compile time.

The canonical Phase 39 contract inventory lives in mailglass_inbound/docs/api_stability.md.

Provider internals, persistence internals, mailbox execution runners, and any operator or UI surfaces remain internal or deferred even when package-local modules exist to support them.