docs: roadmap tâches 1.9-8 (briefs, gates de validation, designs tâche 2) + plans d'implémentation
Cartographie complète (liste_taches/coherence_taches), briefs tacheN + gates validation_tacheN, design tâche 2 (docs/design/tache2/), specs/plans jalon 1-2 et tâche 1.9/2 (Phase 1, Phase 2, SJ-0→3). Validations consignées (1.9 ✅, 2-8 🟡). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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