Accept MIL-003 and add the Play a Game use case

Create the use case chain for the game loop task: Use Case Diagram
UCD-001, User Stories US-001 (two stories), Use Case UC-001 and System
Sequence Diagram SSD-001, all Proposed. Link them from MIL-003.

Review MIL-003 (RC-006, Go) and accept it. Add the new artifacts to the
Traceability Matrix and register UCD, US, UC and SSD in the artifact
registry.

Refs #13

docs/use-case-diagram.md

@@ -0,0 +1,54 @@
+# Use Case Diagram
+
+## Metadata
+| Key | Value |
+| --- | --- |
+| ID | UCD-001 |
+| CrossReference | [SA-001], [BC-001], [US-001], [UC-001] |
+
+## Version History
+| Date | Status | Author | Reviewer | Change | Commit |
+| --- | --- | --- | --- | --- | --- |
+| 2026-10-04 | Proposed | Jens Tirsvad Nielsen | S01 | Initial version | pending |
+
+---
+
+## Purpose and Scope
+
+The system boundary is the Higher Lower console game. One actor, the Player, uses it to play a game. Traces to [BC-001] objective O1 and to stakeholders S01 and S02 ([SA-001]), who play the game; S03 only reads the repository.
+
+## Diagram
+
+```plantuml
+@startuml
+left to right direction
+actor Player <<Actor>>
+rectangle "Higher Lower <<System>>" {
+  usecase "Play a Game" as PlayAGame
+}
+Player --> PlayAGame
+@enduml
+```
+
+## Actor Table
+
+| Actor | Stereotype | Stakeholder ID (SA) | Goals (use cases) |
+| --- | --- | --- | --- |
+| Player | Actor | S01, S02 | Play a Game |
+
+## Use Case Table
+
+| Use Case | Actor(s) | Goal |
+| --- | --- | --- |
+| Play a Game ([UC-001]) | Player | Guess, round after round, which of two accounts has more followers, and see the final score |
+
+## Relationships
+
+None: there is a single use case, so no `<<include>>` or `<<extend>>` is used.
+
+---
+
+[SA-001]: ./stakeholder-analysis.md
+[BC-001]: ./business-case.md
+[US-001]: ./user-stories.md
+[UC-001]: ./uc-001/uc.md