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

deny.toml

@@ -0,0 +1,36 @@
+# cargo-deny — the licence policy of ADR 0002 and ADR 0003, enforced.
+#
+# The whole competitive position is "permissively licensed". A copyleft
+# dependency sneaking in via a transitive bump would quietly destroy it, and
+# nobody would notice until a lawyer did. So CI fails on it.
+
+[licenses]
+version = 2
+allow = [
+    "Apache-2.0",
+    "MIT",
+    "MIT-0",
+    "BSD-2-Clause",
+    "BSD-3-Clause",
+    "ISC",
+    "Unicode-3.0",
+    "Zlib",
+    "CC0-1.0",
+    "MPL-2.0",           # file-level copyleft; acceptable as a leaf dependency
+    "Apache-2.0 WITH LLVM-exception",
+]
+confidence-threshold = 0.9
+
+# Everything not in `allow` fails — including every GPL, LGPL and AGPL variant.
+# That is the point; do not add an exception without amending ADR 0002.
+
+[bans]
+multiple-versions = "warn"
+
+[advisories]
+version = 2
+yanked = "deny"
+
+[sources]
+unknown-registry = "deny"
+unknown-git = "deny"