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.

Commencez par src/lib/wcif/validation/eventRoundValidation.ts et examinez comment les types de @wca/helpers, les mocks internes et les builders représentent les rounds. Comparez les références aux rounds liés et déterminez la forme exacte de WCIF/API avant de suivre le rendu des rounds, les champs CSV et les imports. Le travail est terminé lorsque la validation, la gestion des rounds et les tests couvrent un WCIF à deux rounds sans erreurs positives à tort ni plantages.

Rédigé par le modèle d'indexation à partir du texte de l'issue.