Rewrite as a Rust, Apache-2.0 workspace

Supersedes the Go + embed-Mox design. The Go tree is removed; its
architecture doc is preserved at docs/archive/ARCHITECTURE-go-embed-mox.md
because its competitive analysis and data model still hold.

Five decisions recorded as ADRs:

  0001  Rust, not Go — accepting ~5,500 lines of protocol code that Mox
        would have given us free, to get the first permissively licensed
        Rust mail server. Costs stated plainly.
  0002  Apache-2.0, not MIT or AGPL — patent grant, trademark, CLA-free
        contribution. Public on GitHub; Gitea stays as the private fallback.
  0003  Stalwart's primitive crates (Apache-2.0/MIT) yes; its AGPL server
        crates never. DANE and MTA-STS sit on the AGPL side of that line,
        which is why we write our own.
  0004  Milestones, reordered: embedded inbound is required at launch.
  0005  Oracle Cloud blocks outbound :25, so direct-to-MX is impossible on
        the launch host. Split delivery is mandatory, not an on-ramp.

Twelve crates in three tiers. Tier 1 (mail-dane, mail-mta-sts, mail-dsn)
is standalone and publishable — no `dane` or `mta-sts` crate exists on
crates.io at all today.

openmail-relay ships the provider table as data, with SES and Oracle from
the start. Oracle's and Resend's SPF includes are deliberately None: a
guessed include turns the DNS check green against a mechanism the provider
does not honour, and mail still fails SPF silently.

cargo check/test/clippy/fmt all green; unsafe_code is forbidden workspace
wide; cargo-deny enforces the licence policy in CI.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JkyvfNJGTshJNE9FtwPLk7

crates/openmail-guard/src/lib.rs

@@ -0,0 +1,30 @@
+//! Inbound abuse gate — the first thing an unauthenticated connection meets.
+//!
+//! Three cheap checks, in increasing cost order, run before a message is
+//! accepted or parsed: connection rate limit, DNSBL lookup, and `iprev`
+//! (forward-confirmed reverse DNS). Ordering is deliberate — never spend a DNS
+//! round trip on a connection a counter can reject.
+//!
+//! Every check returns a [`Judgement`] rather than a bool, because "we could
+//! not tell" (DNS timeout) must not be silently equivalent to "clean".
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum Judgement {
+    Clean,
+    /// Reject now, with this SMTP response.
+    Reject {
+        code: u16,
+        text: String,
+    },
+    /// Accept but weight toward junk.
+    Suspicious(String),
+    /// The check itself failed. Fail *open* for DNS errors — a resolver outage
+    /// must not become a mail outage — but record it.
+    Indeterminate(String),
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum Error {
+    #[error("resolver error: {0}")]
+    Resolver(String),
+}