Phase 3: /mcmapper link command and two-way chat bridge

Test-first from here on (per request after Phase 2's backend commit):
LinkCodeGenerator has a standalone test (common/src/test) written and
confirmed failing before the implementation existed.

common: LinkCodeGenerator produces a 6-character code from an
unambiguous charset (excludes 0/O/1/I/L — it gets read off a chat line
and typed back). BackendConnection grows sendLinkRequest (now actually
implemented, was a Phase 1 stub), sendChatMessage, and setChatListener/
ChatListener for inbound web->game chat, all wired into
DefaultBackendConnection's existing JSON wire protocol.

forge-1_12_2: LinkCommand (/mcmapper link) generates a code, resolves
online/offline from the server's actual auth mode
(MinecraftServer#isServerInOnlineMode), and shows it to the player.
Forge1122ChatBridge implements the inbound half (injects web chat into
real in-game chat via the player list) — injectWaypointShare is a
documented Phase 4 stub, same pattern as sendLinkRequest was in Phase 1.
MCMapperMod hooks ServerChatEvent to forward in-game chat out and wires
the chat listener to the bridge.

Verified end-to-end against a live MCMapper-Backend instance through the
real Java client (not a stand-in): a mod-generated link code correctly
redeems via the backend's HTTP endpoint to an account with the mod-
supplied username, and a browser chat message correctly round-trips all
the way to the mod's live ChatListener callback.

forge-1_12_2/src/main/java/com/octoturge/mcmapper/forge1122/Forge1122ChatBridge.java

@@ -0,0 +1,29 @@
+package com.octoturge.mcmapper.forge1122;
+
+import com.octoturge.mcmapper.common.ChatBridge;
+import com.octoturge.mcmapper.common.protocol.WaypointShare;
+import net.minecraft.server.MinecraftServer;
+import net.minecraft.util.text.TextComponentString;
+import org.apache.logging.log4j.Logger;
+
+public class Forge1122ChatBridge implements ChatBridge {
+    private final MinecraftServer server;
+    private final Logger logger;
+
+    public Forge1122ChatBridge(MinecraftServer server, Logger logger) {
+        this.server = server;
+        this.logger = logger;
+    }
+
+    @Override
+    public void injectWebChatMessage(String displayName, String message) {
+        server.getPlayerList().sendMessage(new TextComponentString("[Web] " + displayName + ": " + message));
+    }
+
+    @Override
+    public void injectWaypointShare(WaypointShare waypoint) {
+        // Marker sharing lands in Phase 4 (needs the JourneyMap/Xaero wire-format research spike
+        // the plan calls for) — nothing calls this yet.
+        logger.warn("injectWaypointShare called before Phase 4's waypoint sharing is implemented — ignoring");
+    }
+}

forge-1_12_2/src/main/java/com/octoturge/mcmapper/forge1122/LinkCommand.java

@@ -0,0 +1,55 @@
+package com.octoturge.mcmapper.forge1122;
+
+import com.octoturge.mcmapper.common.BackendConnection;
+import com.octoturge.mcmapper.common.LinkCodeGenerator;
+import com.octoturge.mcmapper.common.protocol.LinkRequest;
+import net.minecraft.command.CommandBase;
+import net.minecraft.command.CommandException;
+import net.minecraft.command.ICommandSender;
+import net.minecraft.entity.player.EntityPlayerMP;
+import net.minecraft.server.MinecraftServer;
+import net.minecraft.util.text.TextComponentString;
+
+/** {@code /mcmapper link} — see LinkRequest.java's javadoc and MCMapper-Backend's link.ts. */
+public class LinkCommand extends CommandBase {
+    private final BackendConnection connection;
+
+    public LinkCommand(BackendConnection connection) {
+        this.connection = connection;
+    }
+
+    @Override
+    public String getName() {
+        return "mcmapper";
+    }
+
+    @Override
+    public String getUsage(ICommandSender sender) {
+        return "/mcmapper link";
+    }
+
+    @Override
+    public int getRequiredPermissionLevel() {
+        return 0; // any player may link their own account
+    }
+
+    @Override
+    public void execute(MinecraftServer server, ICommandSender sender, String[] args) throws CommandException {
+        if (args.length == 0 || !"link".equals(args[0])) {
+            sender.sendMessage(new TextComponentString("Usage: /mcmapper link"));
+            return;
+        }
+        if (!(sender instanceof EntityPlayerMP)) {
+            sender.sendMessage(new TextComponentString("Only players can link an account."));
+            return;
+        }
+
+        EntityPlayerMP player = (EntityPlayerMP) sender;
+        String code = LinkCodeGenerator.generate();
+        LinkRequest.AuthMode authMode =
+                server.isServerInOnlineMode() ? LinkRequest.AuthMode.ONLINE : LinkRequest.AuthMode.OFFLINE;
+        connection.sendLinkRequest(new LinkRequest(player.getUniqueID().toString(), player.getName(), code, authMode));
+        player.sendMessage(new TextComponentString(
+                "Your MCMapper link code: " + code + " — enter it on the map site within 10 minutes."));
+    }
+}

forge-1_12_2/src/main/java/com/octoturge/mcmapper/forge1122/MCMapperMod.java

@@ -5,9 +5,11 @@ import com.octoturge.mcmapper.common.DefaultBackendConnection;
 import com.octoturge.mcmapper.common.config.MapperConfig;
 import com.octoturge.mcmapper.common.protocol.DeltaEvent;
 import com.octoturge.mcmapper.common.protocol.SectionData;
+import net.minecraft.entity.player.EntityPlayerMP;
 import net.minecraft.world.WorldServer;
 import net.minecraftforge.common.MinecraftForge;
 import net.minecraftforge.common.config.Configuration;
+import net.minecraftforge.event.ServerChatEvent;
 import net.minecraftforge.fml.common.Mod;
 import net.minecraftforge.fml.common.event.FMLPreInitializationEvent;
 import net.minecraftforge.fml.common.event.FMLServerStartingEvent;
@@ -27,8 +29,8 @@ import java.util.Set;
  * Entry point for the 1.12.2 leaf — the primary/first-implemented target (Enigmatica 2).
  * Phase 1: connects to the backend over WS, backfills the overworld's loaded chunks, and
  * flushes event-driven column deltas on a timer (no periodic reconciliation sweep yet — see
- * plan's Phase 7). Phase 2 adds full-section backfill/flush for 3D mesh rendering. Chat bridge
- * and `/mcmapper link` land in Phase 3.
+ * plan's Phase 7). Phase 2 adds full-section backfill/flush for 3D mesh rendering. Phase 3 adds
+ * `/mcmapper link` and the two-way chat bridge (see LinkCommand, Forge1122ChatBridge).
  */
 @Mod(modid = MCMapperMod.MOD_ID, name = "MCMapper", version = MCMapperMod.VERSION)
 public class MCMapperMod {
@@ -88,9 +90,19 @@ public class MCMapperMod {
             }
         });
 
+        Forge1122ChatBridge chatBridge = new Forge1122ChatBridge(event.getServer(), LOGGER);
+        connection.setChatListener(chatBridge::injectWebChatMessage);
+        event.registerServerCommand(new LinkCommand(connection));
+
         MinecraftForge.EVENT_BUS.register(this);
     }
 
+    @SubscribeEvent
+    public void onServerChat(ServerChatEvent event) {
+        EntityPlayerMP player = event.getPlayer();
+        connection.sendChatMessage(player.getUniqueID().toString(), player.getName(), event.getMessage());
+    }
+
     @SubscribeEvent
     public void onServerTick(TickEvent.ServerTickEvent event) {
         if (event.phase != TickEvent.Phase.END) return;