Thin router DSL for compiling inbound mailbox routes into pure route data.

A router module declares ordered route/2 entries and exposes the compiled routes through __mailglass_inbound_routes__/0. Runtime matching is handled by MailglassInbound.Router.Matcher, which preserves top-to-bottom, first-match-wins semantics.

Phase 39 intentionally keeps the public matcher surface narrow:

• :recipient accepts an exact string or regex and matches the envelope recipient.
• :subject accepts an exact string or regex.
• :headers accepts {header_name, exact_string_or_regex} tuples.