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

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+# 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

docs/uc-001/uc.md

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+# UC-001 Play a Game
+
+## Metadata
+| Key | Value |
+| --- | --- |
+| ID | UC-001 |
+| CrossReference | [UCD-001], [US-001], [SA-001], [SSD-001] |
+
+## Version History
+| Date | Status | Author | Reviewer | Change | Commit |
+| --- | --- | --- | --- | --- | --- |
+| 2026-10-04 | Proposed | Jens Tirsvad Nielsen | S01 | Initial version | pending |
+
+---
+
+Format: Fully Dressed
+
+## Scope
+
+The Higher Lower console game.
+
+## Level
+
+User goal.
+
+## Primary Actor
+
+Player.
+
+## Stakeholders and Interests
+
+| Stakeholder | Interest |
+| --- | --- |
+| S01 | A correct game that shows good practice ([SA-001]) |
+| S02 | Readable behaviour that matches the assignment ([SA-001]) |
+
+## Preconditions
+
+The Player has started the game.
+
+## Postconditions (Success Guarantee)
+
+The Player has seen the final score of the game.
+
+## Main Success Scenario
+
+1. The Player starts a game.
+2. The System shows two different accounts, A and B.
+3. The Player guesses which account has more followers.
+4. The System tells the Player the guess was right, adds one to the score, and shows the new score.
+5. The System makes the account the Player compared against the new A and shows a new B.
+6. The Player repeats from step 3 until a guess is wrong.
+7. The System tells the Player the guess was wrong and shows the final score.
+
+## Extensions / Alternative Flows
+
+- 3a. The Player gives an answer that is not a valid choice: the System asks again; the round does not change.
+- 3b. Both accounts have the same number of followers: either choice counts as right.
+- 5a. No account is left that differs from the current A: the System ends the game and shows the final score.
+
+No `<<include>>` or `<<extend>>` use cases apply.
+
+## Special Requirements / Business Rules
+
+- Step 2 and 5: A and B are never the same account.
+- Step 3b: equal follower counts make both choices right.
+
+## Open Issues
+
+None.
+
+---
+
+[UCD-001]: ../use-case-diagram.md
+[US-001]: ../user-stories.md
+[SA-001]: ../stakeholder-analysis.md
+[SSD-001]: ./ssd.md