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

.github/workflows/ci.yml

@@ -0,0 +1,55 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+  workflow_dispatch:
+
+env:
+  CARGO_TERM_COLOR: always
+  RUSTFLAGS: -D warnings
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  check:
+    name: check · test · clippy · fmt
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          components: rustfmt, clippy
+      - uses: Swatinem/rust-cache@v2
+      - run: cargo fmt --all --check
+      - run: cargo clippy --workspace --all-targets --all-features
+      - run: cargo test --workspace --all-features
+
+  licences:
+    name: licence policy (ADR 0002)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+      - uses: EmbarkStudios/cargo-deny-action@v2
+        with:
+          command: check licenses bans sources advisories
+
+  # Native aarch64 build on spark-1. Oracle Cloud's free tier is Ampere A1
+  # (aarch64), so the release artefact must be aarch64 — and spark-1 (GB10,
+  # 20 cores, 121 GB) builds it natively rather than cross-compiling.
+  aarch64:
+    name: release build · aarch64 (spark-1)
+    runs-on: [self-hosted, linux, ARM64, spark-1]
+    if: github.ref == 'refs/heads/main' || github.event_name == 'workflow_dispatch'
+    steps:
+      - uses: actions/checkout@v5
+      - uses: Swatinem/rust-cache@v2
+      - run: cargo build --release --workspace
+      - uses: actions/upload-artifact@v4
+        with:
+          name: openmail-aarch64
+          path: target/release/openmail
+          if-no-files-found: error