feat(rust): scaffold Rust workspace and fix TypeScript build errors

- Add @git.zone/tsrust with linux_amd64/linux_arm64 cross-compilation
- Scaffold Rust workspace with 5 crates: mailer-core, mailer-smtp, mailer-security, mailer-napi, mailer-bin
- Fix all TypeScript compilation errors for upgraded dependencies (smartfile v13, mailauth v4.13, smartproxy v23)
- Replace smartfile.fs/memory with @push.rocks/smartfs throughout codebase
- Fix .ts import extensions to .js for NodeNext module resolution
- Update DKIMSignOptions usage to match mailauth v4.13 API
- Add MTA error classes with permissive signatures for legacy SMTP client
- Replace removed DcRouter/StorageManager/deliverability imports with local interfaces

readme.hints.md

@@ -0,0 +1,25 @@
+# Project Hints
+
+## Rust Workspace
+- Rust code lives in `rust/` with a Cargo workspace
+- Crates: `mailer-core`, `mailer-smtp`, `mailer-security`, `mailer-napi`, `mailer-bin`
+- `mailer-bin` is the binary target that `tsrust` builds for cross-compilation
+- `mailer-napi` is a cdylib for N-API bindings (not built by tsrust, needs separate napi-rs build pipeline)
+- tsrust only supports binary targets (looks for `src/main.rs` or `[[bin]]` entries)
+- Cross-compilation targets: `linux_amd64`, `linux_arm64` (configured in `npmextra.json`)
+- Build output goes to `dist_rust/`
+
+## Build
+- `pnpm build` runs `tsbuild tsfolders && tsrust`
+- Note: `tsbuild tsfolders` requires a `tsconfig.json` at the project root (currently missing - pre-existing issue)
+- `tsrust` independently works and produces binaries in `dist_rust/`
+
+## Key Dependencies (Rust)
+- `tokio` - async runtime
+- `tokio-rustls` - TLS
+- `hickory-dns` (hickory-resolver) - DNS resolution
+- `mail-auth` - DKIM/SPF/DMARC (by Stalwart Labs)
+- `mailparse` - MIME parsing
+- `napi` + `napi-derive` - Node.js bindings
+- `ring` - crypto primitives
+- `dashmap` - concurrent hash maps