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:
+              - Requirements_Load_WithMissingChild_ReportsError

src/DemaConsulting.ReqStream/Modeling/RequirementsLoader.cs

@@ -707,6 +707,15 @@ private static void ValidateCyclesFrom(
         {
             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/Tracing/TestMissingChild.cs

@@ -0,0 +1,65 @@
+// Copyright (c) 2026 DEMA Consulting
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+// 
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+// 
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+using DemaConsulting.ReqStream.Modeling;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace DemaConsulting.ReqStream.Tests.Tracing;
+
+/// <summary>
+///     Tests verifying that requirements with missing child references are reported as errors.
+/// </summary>
+[TestClass]
+public class TestMissingChildRequirement
+{
+    /// <summary>
+    ///     Verifies that loading a requirements file where a requirement references a
+    ///     non-existent child ID produces an error-level lint issue.
+    /// </summary>
+    [TestMethod]
+    public void Requirements_Load_WithMissingChild_ReportsError()
+    {
+        var yaml = @"---
+sections:
+  - title: ""Test""
+    requirements:
+      - id: ""PARENT""
+        title: ""Parent requirement""
+        children:
+          - ""NONEXISTENT""
+";
+        var path = Path.Combine(Path.GetTempPath(), $"test_{Guid.NewGuid()}.yaml");
+        File.WriteAllText(path, yaml);
+        try
+        {
+            var result = Requirements.Load(path);
+
+            Assert.IsTrue(result.HasErrors, "Expected errors for missing child reference");
+            Assert.IsNull(result.Requirements, "Requirements should be null when errors exist");
+            Assert.IsTrue(
+                result.Issues.Any(i => i.Severity == LintSeverity.Error && i.Description.Contains("NONEXISTENT")),
+                "Expected an error mentioning the unknown child 'NONEXISTENT'");
+        }
+        finally
+        {
+            File.Delete(path);
+        }
+    }
+}