WCIF/Reg 2026: Support Dual Rounds (linked rounds)

WCA 2026 regs introduce **Dual Rounds** (two consecutive rounds linked together for progression/ranking rules). DelegateDashboard needs to handle competitions using this feature.

Reference:
- WCA announcement: https://www.worldcubeassociation.org/posts/wca-regulations-january-2026 (Major change: Dual Rounds)
- Regulations PR: https://github.com/thewca/wca-regulations-january-2026/pull/50
- WCA site implementation uses a *linked rounds* model. In the WCA website repo, schedule UI receives a `linkedRounds` map keyed by WCIF round id, and the live results API includes `linked_round_ids` on rounds.
- Example schema: https://github.com/thewca/worldcubeassociation.org/blob/master/next-frontend/openapi/schemas/live/LiveRound.yaml

## Likely impacts in DelegateDashboard
1) **Round validation**
- `src/lib/wcif/validation/eventRoundValidation.ts` currently requires `advancementCondition` on every non-final round.
- Dual rounds may need an exception (e.g., round1 of a linked pair may be allowed to omit/alter advancementCondition), depending on how WCIF encodes the link.

2) **UI / exports / imports**
- Anywhere we render a round name/format/proceed text should be able to indicate dual rounds and/or treat linked rounds as a combined unit.
- CSV export fields `advancement_condition` and round listings may need to incorporate the dual-round semantics.

3) **Data model/types**
- Confirm how the WCIF (or WCA API WCIF wrapper) represents linked/dual rounds. If the WCIF schema adds fields, update `@wca/helpers` types usage and our internal mocks/builders.

## Acceptance criteria
- Identify the exact WCIF/API shape for dual rounds (field names + where they live).
- Update validation so dual rounds do not trigger false-positive errors.
- Ensure any round lists/exports/imports handle the linked rounds without crashes.
- Add tests covering a dual-round example WCIF.

src/lib/wcif/validation/eventRoundValidation.ts から始め、@wca/helpers の型、内部モック、builder がラウンドをどのように表現しているかを調べてください。リンクされたラウンドへの参照を比較し、ラウンドのレンダリング、CSV フィールド、インポートを追跡する前に、正確な WCIF/API の形を特定してください。検証、ラウンド処理、テストが、誤検知エラーやクラッシュなしに 2 ラウンドの WCIF をカバーすれば完了です。

索引モデルが issue の本文から書いたものです。