Add incomplete Minecraft 26.3 port prototype

ports/mc263/README.md

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+# Minecraft 26.3 port workspace
+
+This is an **incomplete mod port** for issues #97–#103. It currently registers
+the Crystal Shard and Crystal Block to verify NeoForge 26.3 loading, block and
+item registration, and modern item resources. It also has a server-only
+runtime Age prototype: the operator-only `rivencraft_port age create <name>
+<seed>`, `list`, and `sample <name>` commands exercise separate vanilla-terrain
+levels with persisted seeds. Authored generators, book binding and client
+dimension synchronization are **not** ported, so it is not playable. Its JAR
+task is disabled. Do not upload output
+from this directory to Gitea, CurseForge, or Modrinth.
+
+From the repository root:
+
+```sh
+./gradlew -p ports/mc263 build
+```
+
+This standalone Gradle build uses Minecraft 26.3, NeoForge 26.3.0.99,
+ModDevGradle 2.0.148, and Java 25. The repository root remains the independent
+Minecraft 1.21.1/Java 21 build:
+
+```sh
+./gradlew build
+```
+
+Continue migrating the actual mod in issues #98–#101. Only enable JAR
+assembly once the dual-version validation in #103 is complete.
+
+The dedicated development server uses `build/run-server`, with its own world
+and properties. Set a nonconflicting `server-port` there before running if
+another Minecraft server is active. The prototype restores all indexed Ages
+during the Overworld load, before initial chunk preparation completes.