Add the plan baseline: business case, stakeholders, plan, design and milestones

Business Case, Stakeholder Analysis, Project Plan, use case chain
(UCD, US, UC, DM, SSD, OC, SD, DCD, DICT), three ADRs, milestones
MIL-001 to MIL-007, the traceability matrix and 22 review records
(autonomous mode, AI as stand-in for S01).

docs/use-case-diagram.md

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+# Use Case Diagram: Flash Card
+
+## Metadata
+| Key | Value |
+| --- | --- |
+| ID | UCD-001 |
+| CrossReference | [SA-001], [BC-001], [US-001], [UC-001] |
+| Language | en |
+| Domain | it |
+
+## Version History
+| Date | Status | Author | Reviewer | Change | Commit |
+| --- | --- | --- | --- | --- | --- |
+| 2026-10-14 | Accepted | Jens Tirsvad Nielsen | AI as stand-in for S01 | Initial version | pending |
+
+---
+
+## Purpose and Scope
+
+The system boundary is the flash card program: the window, the word list it reads, the progress file it writes and the optional spoken word. The Learner is the only actor. The git host, the course and the repository's readers are outside the boundary.
+
+## Diagram
+
+```plantuml
+@startuml
+left to right direction
+actor "Learner" as L
+rectangle "Flash Card" <<System>> {
+  usecase "Study Flash Cards" as UC1
+}
+L --> UC1
+@enduml
+```
+
+## Actor Table
+
+| Actor | Stereotype | Stakeholder ID (SA) | Goals (use cases) |
+| --- | --- | --- | --- |
+| Learner | `<<Actor>>` | S01, S02 | Study Flash Cards |
+
+## Use Case Table
+
+| Use Case | Actor(s) | Goal |
+| --- | --- | --- |
+| Study Flash Cards | Learner | Learn the words of a word list by seeing each card, checking its translation and keeping only the words not yet known |
+
+## Relationships
+
+None. Hearing the Malay word is an extension flow of Study Flash Cards (see [UC-001]), not a use case of its own, because it has no goal apart from studying.
+
+---
+
+[SA-001]: ./stakeholder-analysis.md
+[BC-001]: ./business-case.md
+[US-001]: ./user-stories.md
+[UC-001]: ./uc-001/uc.md