feat(diagram-engine): flowcharts, swimlanes, sequence diagrams and mind maps

Extends the declarative engine past cloud architecture. The tool routing was
divided by icon library — AWS through the engine, everything else hand-written
XML — which is the wrong axis. What matters is the LAYOUT SHAPE.

Measured first: a six-step approval flow declared in its natural order comes out
as one column, because the layout only arranges what nesting tells it to and
never looked at the arrows. That forces the arrow from the decision to its second
branch to jump over the first branch.

graph.ts computes what the layout should have looked at: layer assignment by
longest path, cycle breaking so a loop is drawn without setting the order, and
barycentre sweeping to cut edge crossings. It emits ordinary container
operations, so layout, routing and round-tripping are unchanged — reaching zero
arrows-through-boxes on a 14-node pipeline and zero crossings on a bipartite
graph whose declared order forces three.

Three new container kinds, each because one layout rule cannot serve them all:

  pool     — swimlanes. Lanes are real cells and each step is parented to its
             band, so dragging a step to another role records the change.
  sequence — participants across the top, one lifeline cell per participant so
             head and line stay together on a drag. Messages bypass the router:
             a message's height IS its order.
  radial   — mind maps and org charts. Children are a flat list and the
             hierarchy comes from the links, because a branch is a box and a box
             cannot hold children.

Flowchart box shapes (diamond, stadium, parallelogram, document) so a reader can
tell a branch from a step.

Two bugs the new tests caught: the duplicate-link guard blocked a sequence
diagram from having two messages between the same pair, and the fallback message
numbering was shared across containers, pushing a second diagram's messages off
its own lifelines.

523 unit tests and 17 diagram e2e tests pass. Every kind verified round-trip
stable to a fixed point, and rendered in a real browser — draw.io keeps the
lifeline shape and the lane markers.

docs/ja/README_JA.md

@@ -94,6 +94,7 @@ https://github.com/user-attachments/assets/b2eef5f3-b335-4e71-a755-dc2e80931979
 -   **ダイアグラム履歴**：すべての変更を追跡する包括的なバージョン管理。AI編集前のダイアグラムの以前のバージョンを表示・復元可能
 -   **インタラクティブなチャットインターフェース**：AIとリアルタイムでコミュニケーションしてダイアグラムを改善
 -   **クラウドアーキテクチャダイアグラムサポート**：クラウドアーキテクチャダイアグラムの生成を専門的にサポート（AWS、GCP、Azure）
+-   **レイアウトの自動計算**：アーキテクチャ図、フローチャート、スイムレーン図／BPMN、シーケンス図、マインドマップ、組織図では、AI は構造だけ——何が何を含むか、何が何を指すか——を指定し、座標・サイズ・矢印の経路はすべてアプリが計算します。コンテナは必ず中身が収まるサイズになり、同じ階層の要素が重なることはなく、矢印は本来通り抜けてしまう図形を避けて引かれます。その後に手で動かしたり色を変えた部分は図の一部として読み戻されるため、次の編集で元に戻されることはありません。
 -   **アニメーションコネクタ**：より良い可視化のためにダイアグラム要素間に動的でアニメーション化されたコネクタを作成
 
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