Expand Rule 3 to cover all NonParallelPlanReason values

Adds human-readable messages for all 25 known reasons. Severity:
- Warning: actionable reasons (UDFs, cursors, table variables, remote
  queries, trace flags, hints, DML OUTPUT, writeback variables)
- Info: passive/environmental (cost below threshold, edition limits,
  memory-optimized tables, upgrade mode, index build edge cases)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: erikdarlingdata

src/PlanViewer.Core/Services/PlanAnalyzer.cs

@@ -133,18 +133,66 @@ private static void AnalyzeStatement(PlanStatement stmt, AnalyzerConfig cfg)
         {
             var reason = stmt.NonParallelPlanReason switch
             {
+                // User/config forced serial
                 "MaxDOPSetToOne" => "MAXDOP is set to 1",
+                "QueryHintNoParallelSet" => "OPTION (MAXDOP 1) hint forces serial execution",
+                "ParallelismDisabledByTraceFlag" => "Parallelism disabled by trace flag",
+
+                // Passive — optimizer chose serial, nothing wrong
                 "EstimatedDOPIsOne" => "Estimated DOP is 1 (the plan's estimated cost was below the cost threshold for parallelism)",
+
+                // Edition/environment limitations
                 "NoParallelPlansInDesktopOrExpressEdition" => "Express/Desktop edition does not support parallelism",
+                "NoParallelCreateIndexInNonEnterpriseEdition" => "Parallel index creation requires Enterprise edition",
+                "NoParallelPlansDuringUpgrade" => "Parallel plans disabled during upgrade",
+                "NoParallelForPDWCompilation" => "Parallel plans not supported for PDW compilation",
+                "NoParallelForCloudDBReplication" => "Parallel plans not supported during cloud DB replication",
+
+                // Query constructs that block parallelism (actionable)
                 "CouldNotGenerateValidParallelPlan" => "Optimizer could not generate a valid parallel plan. Common causes: scalar UDFs, inserts into table variables, certain system functions, or OPTION (MAXDOP 1) hints",
-                "QueryHintNoParallelSet" => "OPTION (MAXDOP 1) hint forces serial execution",
+                "TSQLUserDefinedFunctionsNotParallelizable" => "T-SQL scalar UDF prevents parallelism. Rewrite as an inline table-valued function, or on SQL Server 2019+ check if the UDF is eligible for automatic inlining",
+                "CLRUserDefinedFunctionRequiresDataAccess" => "CLR UDF with data access prevents parallelism",
+                "NonParallelizableIntrinsicFunction" => "Non-parallelizable intrinsic function in the query",
+                "TableVariableTransactionsDoNotSupportParallelNestedTransaction" => "Table variable transaction prevents parallelism. Consider using a #temp table instead",
+                "UpdatingWritebackVariable" => "Updating a writeback variable prevents parallelism",
+                "DMLQueryReturnsOutputToClient" => "DML with OUTPUT clause returning results to client prevents parallelism",
+                "MixedSerialAndParallelOnlineIndexBuildNotSupported" => "Mixed serial/parallel online index build not supported",
+                "NoRangesResumableCreate" => "Resumable index create cannot use parallelism for this operation",
+
+                // Cursor limitations
+                "NoParallelCursorFetchByBookmark" => "Cursor fetch by bookmark cannot use parallelism",
+                "NoParallelDynamicCursor" => "Dynamic cursors cannot use parallelism",
+                "NoParallelFastForwardCursor" => "Fast-forward cursors cannot use parallelism",
+
+                // Memory-optimized / natively compiled
+                "NoParallelForMemoryOptimizedTables" => "Memory-optimized tables do not support parallel plans",
+                "NoParallelForDmlOnMemoryOptimizedTable" => "DML on memory-optimized tables cannot use parallelism",
+                "NoParallelForNativelyCompiledModule" => "Natively compiled modules do not support parallelism",
+
+                // Remote queries
+                "NoParallelWithRemoteQuery" => "Remote queries cannot use parallelism",
+                "NoRemoteParallelismForMatrix" => "Remote parallelism not available for this query shape",
+
                 _ => stmt.NonParallelPlanReason
             };
 
-            // Warn when the user forced serial or something in the query blocks parallelism.
-            // Info only for passive reasons (cost below threshold, edition limitation).
-            var isActionable = stmt.NonParallelPlanReason is "MaxDOPSetToOne"
-                or "QueryHintNoParallelSet" or "CouldNotGenerateValidParallelPlan";
+            // Actionable: user forced serial, or something in the query blocks parallelism
+            // that could potentially be rewritten. Info: passive (cost too low) or
+            // environmental (edition, upgrade, cursor type, memory-optimized).
+            var isActionable = stmt.NonParallelPlanReason is
+                "MaxDOPSetToOne" or "QueryHintNoParallelSet" or "ParallelismDisabledByTraceFlag"
+                or "CouldNotGenerateValidParallelPlan"
+                or "TSQLUserDefinedFunctionsNotParallelizable"
+                or "CLRUserDefinedFunctionRequiresDataAccess"
+                or "NonParallelizableIntrinsicFunction"
+                or "TableVariableTransactionsDoNotSupportParallelNestedTransaction"
+                or "UpdatingWritebackVariable"
+                or "DMLQueryReturnsOutputToClient"
+                or "NoParallelCursorFetchByBookmark"
+                or "NoParallelDynamicCursor"
+                or "NoParallelFastForwardCursor"
+                or "NoParallelWithRemoteQuery"
+                or "NoRemoteParallelismForMatrix";
 
             stmt.PlanWarnings.Add(new PlanWarning
             {