docs: add onion messaging todo list

Co-authored-by: aider (gemini/gemini-3-pro-preview) <[email protected]>

Author: gijswijs

TODO_ONION_MESSAGING.md

@@ -0,0 +1,51 @@
+# Onion Messaging Implementation Review
+
+## `onionmessage/hop.go`
+
+- [ ] **Refactor `processOnionMessage`**:
+  The current implementation uses a chain of `fn.AndThen` which makes accessing intermediate results (like `processedPkt` and `payload`) in the final step awkward, necessitating `UnwrapOr(nil)`.
+  **Suggestion**: Rewrite `processOnionMessage` using standard Go imperative error handling (`if err != nil`) to improve readability and avoid unsafe unwraps, or introduce a context struct passed through the pipeline.
+
+- [ ] **Magic Value in `processOnionMessagePacket`**:
+  There is a TODO regarding the magic value `10` for `incomingCltv`.
+  ```go
+  // TODO(gijs): We should not use the magic value 10 here.
+  ```
+  **Action**: Resolve this TODO, possibly by defining a constant or determining if a specific value is actually required by the sphinx router for this packet type.
+
+- [ ] **Documentation**:
+  Add GoDoc comments for `ForwardAction`, `DeliverAction`, and `RoutingDecision` types.
+
+## `onionmessage/endpoint.go`
+
+- [ ] **Constructor Validation**:
+  `NewOnionEndpoint` does not validate that `receptionist` or `router` are non-nil.
+  **Suggestion**: Add checks or ensure these are mandatory.
+
+- [ ] **Error Handling in `forwardMessage`**:
+  The error message `no actors found for nextNodeId` could be more descriptive (e.g., is the peer connected?).
+
+## `onionmessage/actor.go`
+
+- [ ] **Exported Fields**:
+  `OMRequest` has an unexported field `msg`. Consider exporting it if external inspection is needed, or keep it unexported if it's strictly internal.
+
+- [ ] **Documentation**:
+  Add comments explaining the role of `OMRequest` and `OMResponse` within the actor system adapter pattern.
+
+## `server.go`
+
+- [ ] **Replay Log TODO**:
+  Resolve the TODO regarding `sphinx.NewMemoryReplayLog()`.
+  ```go
+  // TODO(gijs): remove the memory replay log once lightning-onion supports it.
+  ```
+  **Action**: Check upstream `lightning-onion` status.
+
+## General
+
+- [ ] **Testing**:
+  Ensure `itest/lnd_onion_message_forward_test.go` covers edge cases like:
+  - Forwarding to a disconnected peer (Actor not found).
+  - Malformed onion packets.
+  - Context cancellation.