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
