docs(design): add Spigot Stealth user stories

design/user-stories/us-001-accumulate-invisibility-and-unlock.md

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+---
+type: User Story
+title: "US-001: Accumulate invisibility time and unlock stealth"
+description: Reward sustained use of directly consumed invisibility potions with the stealth ability.
+status: backlog
+---
+
+# US-001: Accumulate invisibility time and unlock stealth
+
+As a **player**, I want my qualifying invisibility time to accumulate so that sustained potion use eventually unlocks stealth.
+
+## Acceptance criteria
+
+- [ ] Only time spent online under an invisibility effect produced by a potion the player directly drank contributes to progression.
+- [ ] Invisibility received from splash potions, lingering potions, tipped arrows, commands, plugins, or other sources does not contribute.
+- [ ] Qualifying time stops when the effect ends, is removed, is replaced by a non-qualifying source, the player disconnects, or the plugin disables.
+- [ ] Refreshed or overlapping qualifying effects never count elapsed time more than once.
+- [ ] Qualifying time accumulates across effects, sessions, and server restarts.
+- [ ] The ability unlocks when accumulated qualifying time reaches eight hours by default.
+- [ ] Reaching the threshold grants the unlock exactly once without discarding excess elapsed time.
+- [ ] When an online player unlocks stealth, they receive both a full-screen title and a chat message explaining the ability.
+- [ ] Progress and unlock ownership are associated with the player's UUID rather than their current name.
+
+## Related
+
+- [Rejoin without a visible identity](us-002-rejoin-without-visible-identity.md)
+- [Check personal stealth progress](us-003-check-personal-progress.md)
+- [Configure and persist stealth progression](us-005-configure-and-persist-progression.md)