checkpoint: implement wgcloak v1 foundations

docs/IMPLEMENTATION_CHECKLIST.md

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+# Implementation and acceptance checklist
+
+This is a coordinator checklist, not a claim that the items are complete.
+Mark an item only with code evidence, tests, and an integration result. Record
+the commit/test command or issue beside each checked item. A feature marked
+planned in the other documents must remain visibly planned here until proven.
+
+## Milestones 1–6
+
+- [ ] **1 — authenticated direct relay:** local UDP listener; direct UDP
+  handshake; HMAC-authenticated data; replay window; keepalive/expiry; one
+  backend UDP socket per authenticated client; multi-client isolation; TOML;
+  logs and basic stats.
+- [ ] **2 — QUIC DATAGRAM:** certificate loading and hostname verification;
+  client challenge/response; datagrams; keepalive/MTU; reconnect; multi-client
+  handling without core API redesign.
+- [ ] **3 — automatic selection:** `mode = "auto"`; direct UDP then QUIC;
+  bounded probes/retries; auth rejection stops probing; jittered reconnect;
+  status reports selected transport.
+- [ ] **4 — HTTP/3 CONNECT-UDP:** standard CONNECT-UDP/HTTP Datagram semantics;
+  authentication; backend mapping; reuse QUIC infrastructure where practical.
+- [ ] **5 — TLS/TCP fallback:** TLS listener; client authentication; bounded
+  big-endian length framing; bidirectional datagram reconstruction; fallback
+  integration; expected head-of-line behavior documented.
+- [ ] **6 — operational polish:** Unix control socket; status/stats/reconnect
+  commands; config validation; systemd units; capability guidance; certificate
+  and configuration reload; benchmarks; fuzzing; installation docs.
+
+## v1 completion criteria (spec §89)
+
+- [ ] 1. Multiple clients concurrently reach one UDP backend.
+- [ ] 2. No WireGuard-specific logic exists.
+- [ ] 3. Direct UDP works reliably.
+- [ ] 4. QUIC DATAGRAM works reliably.
+- [ ] 5. HTTP/3 CONNECT-UDP works.
+- [ ] 6. TLS/TCP fallback works.
+- [ ] 7. Automatic transport selection works.
+- [ ] 8. Authentication is enforced on every public transport.
+- [ ] 9. Direct UDP has replay and integrity protection.
+- [ ] 10. Direct UDP NAT rebinding works.
+- [ ] 11. Sessions expire and clean up correctly.
+- [ ] 12. The server survives malformed and unauthenticated traffic.
+- [ ] 13. The daemon runs unprivileged except optional bind capability.
+- [ ] 14. TOML configuration is clearly validated.
+- [ ] 15. Meaningful status and statistics are available.
+- [ ] 16. Reconnects occur automatically.
+- [ ] 17. Automated integration tests cover multi-client operation.
+
+## Required evidence before v1 sign-off
+
+- [ ] Unit tests cover parsing, malformed input, auth, key derivation, replay,
+  config validation, and timeout logic.
+- [ ] Integration tests cover direct client/server, concurrent clients,
+  source-port change, auth rejection, restart/reconnect, backend outage,
+  oversized datagrams, and expiry; later transports have equivalent coverage.
+- [ ] Fuzz targets exercise direct packets, TCP frames, and auth messages.
+- [ ] `cargo test`, formatting, linting, and any documented integration/bench
+  commands pass from a clean checkout.
+- [ ] Review confirms no payload/secret logging, no unsafe privilege grant, and
+  no accidental WireGuard/routing/firewall coupling.