Paper 26.2 bundles Adventure 5.2.0, a major bump from the 4.26.1 shipped by 26.1.
Adventure 5 makes ClickEvent generic, so the raw type in
CommandContext.replyWithEvent no longer compiled. Auditing the rest of the
Adventure 5 removals found no other affected usage.

api-version is raised to 26.2, dropping 26.1 support: the two Adventure majors
are not binary compatible, so claiming 26.1 compatibility would be a lie.

Also fixes two problems found while verifying on a real server: the Velocity
healthcheck invoked mc-health, which does not exist in the mc-proxy image, so the
container had never once passed its check; and the debug environments pinned no
build at all, which meant the proxy came up as 4.1.0-SNAPSHOT while the plugin is
compiled against velocity-api 3.5.1. Server versions are now derived by x from
the Gradle coordinates so they cannot drift from what the plugin targets.

マルチプロジェクト構造に移行し，Paper と Velocity の両方をサポートできるようにしました．

- engine: 共通ロジック（データモデル，JSON シリアライゼーション）
- platform-paper: Paper プラグイン実装
- platform-velocity: Velocity プラグイン実装（将来用）

主な変更:
- settings.gradle.kts: サブプロジェクト定義を追加
- build.gradle.kts: マルチプロジェクト用に再構成
- 既存コードを platform-paper に移動
- CI/CD ワークフローを更新
- Docker 設定を更新
- IntelliJ Run Configurations を更新

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>