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

api/src/ws-gateway.ts

@@ -41,6 +41,14 @@ import { recordAndPublishChat } from "./chat";
 // viewers, see chat.ts); api -> mod is sent by the browser-facing /ws/chat route (chat-gateway.ts)
 // looking up the mod's connection via getModSocket() and calling ChatBridge.injectWebChatMessage
 // on the mod side.
+//
+//   api -> mod  {"type":"waypoint_share","name":"...","x":..,"y":..,"z":..,"dimension":..,"color":"#RRGGBB","format":"journeymap"|"xaero"}
+//
+// Phase 4: sent by markers.ts's shareMarkerToChat() via getModSocket() when a linked account
+// shares a placed marker to chat. The api only resolves *which* format the target server wants
+// (per-server `waypointFormat` config) and forwards the structured point — the mod owns building
+// the actual chat text in that format (see WaypointShare.java's javadoc for the researched wire
+// formats and their attribution).
 
 interface Column {
   x: number;