chore: save session checkpoint (Phase 5c complete)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Oliver Baer

.memory/letter_20260206_0008.md

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+# Letter to Myself (Session Handoff)
+
+**Date:** 2026-02-06 (session 8)
+
+## 1. Executive Summary
+* **Goal:** Complete Phase 5c cross-network WebRTC sync with signaling relay, including E2E test suite for the signaling server.
+* **Current Status:** Phase 5c fully implemented and committed. Signaling server E2E tests written, passing, committed (`997e6a7`), and pushed to remote. All Phase 5 work (5a/5b/5d/5c) is complete and on `main`.
+
+## 2. The "Done" List (Context Anchor)
+* **Phase 5c implemented and committed** (`c564257`):
+  - `signaling-server/src/main.rs` — standalone axum 0.8 WebSocket relay (~200 lines)
+  - `src-tauri/src/sync/signaling.rs` — SignalingClient with SHA-256 room IDs, base64 payloads
+  - `src-tauri/src/sync/webrtc.rs` — datachannel-rs adapters (DataChannelStream/Sink), WebRTC negotiation flow
+  - Modified `src-tauri/src/sync/mod.rs` — transport auto-selection (mDNS 5s → WebRTC fallback)
+  - Modified `src-tauri/src/sync/transport.rs` — 5 functions changed to `pub(crate)`
+  - Added deps: `datachannel = "0.16"`, `webrtc-sdp = "0.3"`, `base64 = "0.22"`
+* **Signaling server E2E tests** (`997e6a7`):
+  - `signaling-server/tests/e2e.rs` — 5 integration tests (`#[tokio::test]`)
+  - Tests: basic relay, peer join/leave, room isolation, room full rejection, full SPAKE2+SDP+ICE flow
+  - Added dev-dependencies: async-tungstenite, futures-util, tokio-rustls
+  - `signaling-server/Cargo.lock` committed
+* **All pushed to remote** (main branch, `997e6a7`)
+* **Test counts:** 43 Rust tests + 53 frontend tests all passing
+
+## 3. The "Pain" Log (CRITICAL)
+* **Tried:** `datachannel-rs` `data_channel_handler` with `DataChannelInit`
+* **Failed:** Wrong type — it takes `DataChannelInfo` (not `DataChannelInit`)
+* **Workaround:** Changed parameter type to `DataChannelInfo`
+
+* **Tried:** Treating `SessionDescription.sdp` as `String`
+* **Failed:** It's `webrtc_sdp::SdpSession`, not `String`
+* **Workaround:** Use `.to_string()` to serialize, `webrtc_sdp::parse_sdp(&str, false)` to deserialize. Must add `webrtc-sdp = "0.3"` as explicit dependency even though datachannel re-exports types.
+
+* **Tried:** axum 0.8 WebSocket `split()` (like tungstenite)
+* **Failed:** No `split()` method on axum 0.8 `WebSocket`
+* **Workaround:** Use `socket.recv()` / `socket.send()` directly in `tokio::select!`
+
+* **Tried:** E2E tests with hardcoded room names
+* **Failed:** Race conditions between parallel tests sharing rooms
+* **Workaround:** `AtomicU32` counter for unique room names per test + 20-50ms sleep between joins
+
+* **Tried:** `drop(tx)` to trigger peer_left notification
+* **Failed:** Didn't send close frame fast enough
+* **Workaround:** Explicitly send `Message::Close(None)` before dropping
+
+## 4. Active Variable State
+* Signaling server binary: `signaling-server/target/debug/aurus-signaling-server`
+* Server port: `0.0.0.0:8765` (hardcoded in main.rs)
+* E2E tests expect server at: `ws://localhost:8765/ws`
+* `DEFAULT_SIGNALING_URL` in `src-tauri/src/sync/mod.rs`: `"ws://localhost:8765/ws"`
+* Git branch: `main`, latest commit: `997e6a7`
+* Commit history: `031c7df` (5a) → `be74ebd` (5b) → `140b81a` (5d) → `c564257` (5c) → `2b2fcef` (drafts) → `997e6a7` (e2e tests)
+
+## 5. Immediate Next Steps
+1. [ ] Deploy signaling server (cloud VM, Railway, or Fly.io) and update `DEFAULT_SIGNALING_URL`
+2. [ ] Real-device testing: two Tauri instances on different networks, pair via code, verify transcript sync over WebRTC
+3. [ ] Add STUN/TURN server configuration (currently hardcoded `stun.l.google.com:19302`)
+4. [ ] Frontend UI for sync (SyncPairing.tsx, SyncStatus.tsx components — referenced in plan but not yet built)
+5. [ ] Consider rate limiting / abuse prevention on signaling server for production