Phase 4: web markers and JourneyMap/Xaero waypoint chat sharing

Linked accounts can place 2D markers on the map (y auto-derived from the
chunk store's heightmap); anonymous visitors keep a localStorage-only
list via the same height lookup. Sharing a marker forwards a structured
payload to the mod over its WS connection, which builds the actual
chat text (see MCMapper-Mod for the JourneyMap/Xaero formatting).

Built test-first per the project's TDD workflow.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015tKdPZt78zbPUZMXWzKEKt

THIRD_PARTY_NOTICES.md

@@ -7,7 +7,14 @@ use. Nothing is copied from closed-source projects — in particular, Xaero's Mi
 closed-source, so any Xaero-compatible waypoint link support is implemented purely from publicly
 documented/community-reverse-engineered wire format, never from Xaero's code.
 
-No third-party code has been adapted yet. Entries will be added here as they land, in the form:
+## JourneyMap / Xaero waypoint chat-link formats
+- This backend only resolves a marker's `y` and forwards a structured `{name,x,y,z,dimension,
+  color,format}` payload to the mod over its WS connection (see `api/src/markers.ts`'s
+  `shareMarkerToChat`) — the actual JourneyMap/Xaero chat-text formats are built entirely on the
+  mod side. See `MCMapper-Mod/THIRD_PARTY_NOTICES.md` for the researched sources and attribution
+  (`common/.../protocol/WaypointChatFormatter.java`).
+
+Further entries will be added here as they land, in the form:
 
 ```
 ## <thing adapted>