docs(design): define creeper aura plugin

design/user-stories/us-003-show-progression-and-rank-advancement.md

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+---
+type: User Story
+title: "US-003: Show progression and rank advancement"
+description: Give players brief kill progress displays and prominent rank-up notifications.
+status: backlog
+---
+
+# US-003: Show progression and rank advancement
+
+As a **player**, I want visible progress and rank-up notifications so that I understand my advancement without persistent screen clutter.
+
+## Acceptance criteria
+
+- [ ] After each qualifying creeper kill, a temporary boss bar shows the player's current state and progress toward the next rank.
+- [ ] A locked player sees progress toward Creeper Aura I.
+- [ ] A ranked player sees their current Roman-numeral rank, current kill total, and the next threshold.
+- [ ] The display duration is configurable and defaults to a short period measured in seconds.
+- [ ] The boss bar is hidden automatically when its display period expires.
+- [ ] A rank VI player no longer sees a progress boss bar.
+- [ ] Each newly attained rank displays a full-screen title naming the rank.
+- [ ] Joining the server does not replay a previously acknowledged rank-up title.
+- [ ] Administrative progress changes update the display state of an online affected player.
+- [ ] When one progress event satisfies multiple ranks, the resulting rank and notification behavior is deterministic and tested.
+
+## Related
+
+- [Creeper Aura ranks](us-002-unlock-creeper-aura-ranks.md)
+- [Check personal progress](us-004-check-personal-progress.md)
+- [User-story catalog](index.md)