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/uc-001/ssd.md

@@ -0,0 +1,53 @@
+# System Sequence Diagram: Play a Game
+
+## Metadata
+| Key | Value |
+| --- | --- |
+| ID | SSD-001 |
+| CrossReference | [UC-001] |
+
+## Version History
+| Date | Status | Author | Reviewer | Change | Commit |
+| --- | --- | --- | --- | --- | --- |
+| 2026-10-04 | Proposed | Jens Tirsvad Nielsen | S01 | Initial version | pending |
+
+---
+
+## Source Use Case
+
+[UC-001] Play a Game, main success scenario. The alternative flows (invalid answer, tie, no account left) are out of scope for this diagram.
+
+## Diagram
+
+```plantuml
+@startuml
+actor Player
+participant ":System" as System
+Player -> System : startGame()
+System --> Player : accounts A and B
+loop while the guess is right
+  Player -> System : makeGuess(choice)
+  System --> Player : right, score, new accounts A and B
+end
+Player -> System : makeGuess(choice)
+System --> Player : wrong, final score
+@enduml
+```
+
+## System Operations Table
+
+| Step | Message (verb phrase) | Parameters | Return | Use case step |
+| --- | --- | --- | --- | --- |
+| 1 | startGame | none | accounts A and B | 1, 2 |
+| 2 | makeGuess | choice (A or B) | right, new score, new accounts A and B | 3, 4, 5, 6 |
+| 3 | makeGuess | choice (A or B) | wrong, final score | 3, 7 |
+
+Steps 2 and 3 are the same operation; the loop shows the right-guess outcome repeating, and the last call shows the wrong-guess outcome.
+
+## Lifecycle Notes
+
+A game starts with `startGame` and ends when `makeGuess` returns wrong; the System keeps the score and the current accounts in between.
+
+---
+
+[UC-001]: ./uc.md