fix: validate missing child requirement references and fix test compilation errors

docs/reqstream/modeling/requirements.yaml

@@ -345,3 +345,16 @@ sections:
               - lint
             tests:
               - Linter_Lint_WithMultipleCycles_ReportsAllCycles
+
+          - id: ReqStream-Lint-UnknownChildReference
+            title: >-
+              The requirements loader shall report an error when a requirement references a child
+              requirement ID that does not exist.
+            justification: |
+              References to non-existent child requirement IDs indicate typos or stale references.
+              Reporting these as errors ensures that requirement hierarchies remain consistent and
+              prevents silent failures in trace matrix generation.
+            tags:
+              - lint
+            tests:
+              - RequirementsLoader_Load_WithUnknownChildReference_ReportsError

src/DemaConsulting.ReqStream/Modeling/RequirementsLoader.cs

@@ -33,19 +33,19 @@
     ///     Known fields at the document root level.
     /// </summary>
     private static readonly HashSet<string> KnownDocumentFields =
         new(StringComparer.Ordinal) { "sections", "mappings", "includes" };
 
     /// <summary>
     ///     Known fields within a section.
     /// </summary>
     private static readonly HashSet<string> KnownSectionFields =
         new(StringComparer.Ordinal) { "title", "requirements", "sections" };
 
     /// <summary>
     ///     Known fields within a requirement.
     /// </summary>
     private static readonly HashSet<string> KnownRequirementFields =
         new(StringComparer.Ordinal) { "id", "title", "justification", "tests", "children", "tags" };
 
     /// <summary>
     ///     Known fields within a test mapping.
@@ -692,7 +692,7 @@
     /// <param name="visiting">IDs currently on the active DFS stack.</param>
     /// <param name="currentPath">Ordered list of IDs on the active DFS path (for cycle reporting).</param>
     /// <param name="visited">IDs already fully processed.</param>
     private static void ValidateCyclesFrom(
         string reqId,
         Dictionary<string, Requirement> allRequirements,
         List<LintIssue> issues,
@@ -707,6 +707,15 @@
         {
             foreach (var childId in requirement.Children)
             {
+                if (!allRequirements.ContainsKey(childId))
+                {
+                    issues.Add(new LintIssue(
+                        requirement.Location ?? reqId,
+                        LintSeverity.Error,
+                        $"Requirement '{reqId}' references unknown child '{childId}'"));
+                    continue;
+                }
+
                 if (visiting.Contains(childId))
                 {
                     var cycleStart = currentPath.IndexOf(childId);

test/DemaConsulting.ReqStream.Tests/Modeling/RequirementsLoadParsingTests.cs

@@ -128,6 +128,10 @@ public void Requirements_Load_RequirementWithChildren_ParsesChildrenCorrectly()
         children:
           - ""AUTH-001""
           - ""AUTH-002""
+      - id: ""AUTH-001""
+        title: ""The system shall validate user credentials.""
+      - id: ""AUTH-002""
+        title: ""The system shall reject invalid credentials.""
 ";
         var filePath = Path.Combine(_testDirectory, "requirements.yaml");
         File.WriteAllText(filePath, yamlContent);

test/DemaConsulting.ReqStream.Tests/Modeling/RequirementsLoaderTests.cs

@@ -838,4 +838,26 @@ public void Linter_Lint_WithMultipleCycles_ReportsAllCycles()
             .Count(line => line.Contains("Circular requirement reference detected"));
         Assert.AreEqual(2, cycleCount, $"Expected exactly 2 cycle errors, got {cycleCount}: {errors}");
     }
+
+    /// <summary>
+    /// Test that a child reference to a non-existent requirement ID is reported as an error.
+    /// </summary>
+    [TestMethod]
+    public void RequirementsLoader_Load_WithUnknownChildReference_ReportsError()
+    {
+        var reqFile = Path.Combine(_testDirectory, "unknown-child.yaml");
+        File.WriteAllText(reqFile, @"sections:
+  - title: Test Section
+    requirements:
+      - id: PARENT
+        title: Parent Requirement
+        children:
+          - NONEXISTENT
+");
+
+        var (exitCode, errors) = RunLint(reqFile);
+
+        Assert.AreEqual(1, exitCode);
+        Assert.Contains("NONEXISTENT", errors);
+    }
 }