Initial commit: standalone quiz-testing system

Full-stack F# (Domain/Server/Client via Fable+Elmish+Feliz), PostgreSQL
persistence via Dapper, Docker Compose deployment. Student quiz-taking flow
with time-limit enforcement and focus-loss tracking, Teacher question bank
and quiz builder with results analytics, Admin user management.

tests/Domain.Tests/ValidationTests.fs

@@ -0,0 +1,98 @@
+module Domain.Tests.ValidationTests
+
+open System
+open Xunit
+open Domain
+
+let private newQuestion (qType: QuestionType) : Question =
+    { Id = QuestionId(Guid.NewGuid())
+      TopicId = TopicId(Guid.NewGuid())
+      Text = "Sample?"
+      Points = 1.0
+      Type = qType }
+
+[<Fact>]
+let ``single choice: correct option must be among the listed options`` () =
+    let optA = { Id = OptionId(Guid.NewGuid()); Text = "A" }
+    let optB = { Id = OptionId(Guid.NewGuid()); Text = "B" }
+    let foreignId = OptionId(Guid.NewGuid())
+    let question = newQuestion (SingleChoice([ optA; optB ], foreignId))
+
+    match QuestionValidation.validate question with
+    | Error errors -> Assert.Contains("Correct option must be one of the provided options", errors)
+    | Ok _ -> Assert.Fail "expected validation error"
+
+[<Fact>]
+let ``multiple choice: correct set must be subset of options`` () =
+    let optA = { Id = OptionId(Guid.NewGuid()); Text = "A" }
+    let optB = { Id = OptionId(Guid.NewGuid()); Text = "B" }
+    let foreignId = OptionId(Guid.NewGuid())
+    let question = newQuestion (MultipleChoice([ optA; optB ], Set.ofList [ optA.Id; foreignId ]))
+
+    match QuestionValidation.validate question with
+    | Error errors -> Assert.Contains("Correct options must be a subset of the provided options", errors)
+    | Ok _ -> Assert.Fail "expected validation error"
+
+[<Fact>]
+let ``numeric: negative tolerance is invalid`` () =
+    let question = newQuestion (Numeric(1.0, -0.5))
+
+    match QuestionValidation.validate question with
+    | Error errors -> Assert.Contains("Tolerance must not be negative", errors)
+    | Ok _ -> Assert.Fail "expected validation error"
+
+[<Fact>]
+let ``valid question passes validation`` () =
+    let optA = { Id = OptionId(Guid.NewGuid()); Text = "A" }
+    let optB = { Id = OptionId(Guid.NewGuid()); Text = "B" }
+    let question = newQuestion (SingleChoice([ optA; optB ], optA.Id))
+
+    match QuestionValidation.validate question with
+    | Ok _ -> ()
+    | Error errors -> Assert.Fail(String.concat "; " errors)
+
+[<Fact>]
+let ``quiz: must contain at least one question`` () =
+    let quiz: Quiz =
+        { Id = QuizId(Guid.NewGuid())
+          OwnerId = UserId(Guid.NewGuid())
+          Title = "Empty quiz"
+          Description = ""
+          TimeLimit = None
+          MaxAttempts = None
+          GradingMethod = HighestAttempt
+          ShuffleQuestions = false
+          ShuffleAnswers = false
+          OpenFrom = None
+          OpenTo = None
+          PassingScore = None
+          QuestionSources = []
+          AssignedStudentIds = Set.empty }
+
+    match QuizValidation.validate quiz with
+    | Error errors -> Assert.Contains("Quiz must contain at least one question", errors)
+    | Ok _ -> Assert.Fail "expected validation error"
+
+[<Fact>]
+let ``quiz: OpenFrom must precede OpenTo`` () =
+    let now = DateTimeOffset.UtcNow
+
+    let quiz: Quiz =
+        { Id = QuizId(Guid.NewGuid())
+          OwnerId = UserId(Guid.NewGuid())
+          Title = "Backwards window"
+          Description = ""
+          TimeLimit = None
+          MaxAttempts = None
+          GradingMethod = HighestAttempt
+          ShuffleQuestions = false
+          ShuffleAnswers = false
+          OpenFrom = Some(now.AddDays 1.0)
+          OpenTo = Some now
+          PassingScore = None
+          QuestionSources = [ FixedQuestion { QuestionId = QuestionId(Guid.NewGuid()); Points = 1.0; Order = 0 } ]
+          AssignedStudentIds = Set.empty }
+
+    match QuizValidation.validate quiz with
+    | Error errors -> Assert.Contains("OpenFrom must be before OpenTo", errors)
+    | Ok _ -> Assert.Fail "expected validation error"