feat(generator): emit SEM001 when relationship references unknown dimension

When dimensions.json declares an integral / derivative / dotProduct /
crossProduct that targets a dimension that doesn't exist (typo, stale name
after a rename), QuantitiesGenerator silently dropped the operator. Typos
were invisible until someone noticed the missing operator at the call site.

Now: emit a Warning-severity SEM001 diagnostic naming the owning dimension,
the unresolved reference, and the metadata path, e.g.

  warning SEM001: Dimension 'Force' references unknown dimension 'Lengt'
  in integrals[Lengt -> Energy].other; the operator will not be generated.
  Check spelling and that the referenced dimension exists in
  dimensions.json.

The operator is still skipped (build doesn't fail) so existing valid trees
keep working, but typos surface in build output where they're noticeable.

Closes #56.

Author: claude

Semantics.SourceGenerators/Generators/QuantitiesGenerator.cs

@@ -20,6 +20,14 @@ namespace Semantics.SourceGenerators;
 [Generator]
 public class QuantitiesGenerator : GeneratorBase<DimensionsMetadata>
 {
+	private static readonly DiagnosticDescriptor UnknownDimensionReference = new(
+		id: "SEM001",
+		title: "Unknown dimension reference in physics relationship",
+		messageFormat: "Dimension '{0}' references unknown dimension '{1}' in {2}; the operator will not be generated. Check spelling and that the referenced dimension exists in dimensions.json.",
+		category: "Semantics.SourceGenerators",
+		defaultSeverity: DiagnosticSeverity.Warning,
+		isEnabledByDefault: true);
+
 	public QuantitiesGenerator() : base("dimensions.json") { }
 
 	protected override void Generate(SourceProductionContext context, DimensionsMetadata metadata, CodeBlocker codeBlocker)
@@ -32,8 +40,8 @@ protected override void Generate(SourceProductionContext context, DimensionsMeta
 		// Phase A: Build maps and collect operators
 		Dictionary<string, PhysicalDimension> dimensionMap = BuildDimensionMap(metadata);
 		Dictionary<string, int> typeFormMap = BuildTypeFormMap(metadata);
-		List<OperatorInfo> allOperators = CollectAllOperators(metadata, dimensionMap);
-		List<ProductInfo> allProducts = CollectAllProducts(metadata, dimensionMap);
+		List<OperatorInfo> allOperators = CollectAllOperators(context, metadata, dimensionMap);
+		List<ProductInfo> allProducts = CollectAllProducts(context, metadata, dimensionMap);
 		Dictionary<string, List<OperatorInfo>> operatorsByOwner = GroupBy(allOperators, o => o.OwnerTypeName);
 		Dictionary<string, List<ProductInfo>> productsByOwner = GroupBy(allProducts, p => p.SelfTypeName);
 
@@ -112,7 +120,7 @@ private static Dictionary<string, int> BuildTypeFormMap(DimensionsMetadata metad
 		return map;
 	}
 
-	private static List<OperatorInfo> CollectAllOperators(DimensionsMetadata metadata, Dictionary<string, PhysicalDimension> dimMap)
+	private static List<OperatorInfo> CollectAllOperators(SourceProductionContext context, DimensionsMetadata metadata, Dictionary<string, PhysicalDimension> dimMap)
 	{
 		HashSet<string> seen = [];
 		List<OperatorInfo> result = [];
@@ -124,11 +132,13 @@ private static List<OperatorInfo> CollectAllOperators(DimensionsMetadata metadat
 			{
 				if (!dimMap.TryGetValue(integral.Other, out PhysicalDimension? otherDim))
 				{
+					ReportUnknownReference(context, dim.Name, integral.Other, $"integrals[{integral.Other} -> {integral.Result}].other");
 					continue;
 				}
 
 				if (!dimMap.TryGetValue(integral.Result, out PhysicalDimension? resultDim))
 				{
+					ReportUnknownReference(context, dim.Name, integral.Result, $"integrals[{integral.Other} -> {integral.Result}].result");
 					continue;
 				}
 
@@ -168,11 +178,13 @@ private static List<OperatorInfo> CollectAllOperators(DimensionsMetadata metadat
 			{
 				if (!dimMap.TryGetValue(derivative.Other, out PhysicalDimension? otherDim))
 				{
+					ReportUnknownReference(context, dim.Name, derivative.Other, $"derivatives[{derivative.Other} -> {derivative.Result}].other");
 					continue;
 				}
 
 				if (!dimMap.TryGetValue(derivative.Result, out PhysicalDimension? resultDim))
 				{
+					ReportUnknownReference(context, dim.Name, derivative.Result, $"derivatives[{derivative.Other} -> {derivative.Result}].result");
 					continue;
 				}
 
@@ -205,7 +217,7 @@ private static List<OperatorInfo> CollectAllOperators(DimensionsMetadata metadat
 		return result;
 	}
 
-	private static List<ProductInfo> CollectAllProducts(DimensionsMetadata metadata, Dictionary<string, PhysicalDimension> dimMap)
+	private static List<ProductInfo> CollectAllProducts(SourceProductionContext context, DimensionsMetadata metadata, Dictionary<string, PhysicalDimension> dimMap)
 	{
 		HashSet<string> seen = [];
 		List<ProductInfo> result = [];
@@ -217,11 +229,13 @@ private static List<ProductInfo> CollectAllProducts(DimensionsMetadata metadata,
 			{
 				if (!dimMap.TryGetValue(dot.Other, out PhysicalDimension? otherDim))
 				{
+					ReportUnknownReference(context, dim.Name, dot.Other, $"dotProducts[{dot.Other} -> {dot.Result}].other");
 					continue;
 				}
 
 				if (!dimMap.TryGetValue(dot.Result, out PhysicalDimension? resultDim))
 				{
+					ReportUnknownReference(context, dim.Name, dot.Result, $"dotProducts[{dot.Other} -> {dot.Result}].result");
 					continue;
 				}
 
@@ -255,11 +269,13 @@ private static List<ProductInfo> CollectAllProducts(DimensionsMetadata metadata,
 			{
 				if (!dimMap.TryGetValue(cross.Other, out PhysicalDimension? otherDim))
 				{
+					ReportUnknownReference(context, dim.Name, cross.Other, $"crossProducts[{cross.Other} -> {cross.Result}].other");
 					continue;
 				}
 
 				if (!dimMap.TryGetValue(cross.Result, out PhysicalDimension? resultDim))
 				{
+					ReportUnknownReference(context, dim.Name, cross.Result, $"crossProducts[{cross.Other} -> {cross.Result}].result");
 					continue;
 				}
 
@@ -297,6 +313,16 @@ private static void AddOp(List<OperatorInfo> list, HashSet<string> seen, string
 		}
 	}
 
+	private static void ReportUnknownReference(SourceProductionContext context, string owningDimension, string unknownReference, string fieldPath)
+	{
+		context.ReportDiagnostic(Diagnostic.Create(
+			UnknownDimensionReference,
+			Location.None,
+			owningDimension,
+			unknownReference,
+			fieldPath));
+	}
+
 	private static Dictionary<string, List<T>> GroupBy<T>(List<T> items, Func<T, string> keySelector)
 	{
 		Dictionary<string, List<T>> groups = [];