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-junk/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "openmail-junk"
+description = "Per-inbox Bayesian spam classifier with trainable, persistable state."
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+repository.workspace = true
+homepage.workspace = true
+authors.workspace = true
+
+[lints]
+workspace = true
+
+[dependencies]
+thiserror.workspace = true
+serde.workspace = true
+sha2.workspace = true

crates/openmail-junk/src/lib.rs

@@ -0,0 +1,26 @@
+//! Per-inbox Bayesian spam classification.
+//!
+//! Per-inbox, not global: an agent mailbox that only ever receives webhook
+//! receipts has a radically different prior than a human's. A shared corpus
+//! makes both worse.
+//!
+//! The classifier state must be persistable and versioned — a model that
+//! cannot be rolled back is a model that can silently start eating real mail.
+
+/// A score in `[0.0, 1.0]`; higher is more likely junk.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct Score(pub f32);
+
+impl Score {
+    /// Conventional threshold. Deliberately not a global constant used for
+    /// filing decisions — the caller owns policy, this crate owns the number.
+    pub const LIKELY_JUNK: f32 = 0.9;
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum Error {
+    #[error("corpus not trained")]
+    Untrained,
+    #[error("state version {found} is not readable by this build (expects {expected})")]
+    VersionMismatch { found: u32, expected: u32 },
+}