Sync PlanAnalyzer and BenefitScorer from PerformanceStudio (Apr 9-16)

Port PS PRs #216, #217, #219, #224, #229, #230, #231 to PM.

PlanAnalyzer changes:
- Rule 5: Suppress for Key Lookups (point lookups mislead per-execution estimates)
- Rule 8: Enhanced parallel skew with batch mode sort detection and practical context
- Rule 9: Large memory grant shows top 3 consumers sorted by row count
- Rule 10: Key lookup overhaul — show output columns, check predicate filtering, softer advice
- Rules 11/12/29: Suppress on 0-execution nodes (operator never ran)
- Rule 11: I/O wait severity elevation when scan hits disk
- Rule 24: FormatNodeRef helper includes object name for data access operators
- Rule 26: Suppress when row goal prediction was correct, specific cause detection
- Wait stats: DescribeWaitType with full wait type coverage, multi-wait summary
- New helpers: GetWaitLabel, HasSignificantIoWaits, IdentifyRowGoalCause, FormatNodeRef
- GetOperatorOwnElapsedMs changed to internal for BenefitScorer access

BenefitScorer (new file):
- Stage 1: MaxBenefitPercent for operator-level rules (filter, spill, lookup, etc.)
- Stage 2: Wait stats benefit scoring with parallel allocation (Joe's formula)

PlanModels additions:
- MaxBenefitPercent and ActionableFix on PlanWarning
- WaitBenefit class and WaitBenefits list on PlanStatement

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

Author: erikdarlingdata

Dashboard/Models/PlanModels.cs

@@ -61,6 +61,7 @@ public class PlanStatement
     public SetOptionsInfo? SetOptions { get; set; }
     public List<PlanParameter> Parameters { get; set; } = new();
     public List<WaitStatInfo> WaitStats { get; set; } = new();
+    public List<WaitBenefit> WaitBenefits { get; set; } = new();
     public QueryTimeInfo? QueryTimeStats { get; set; }
 
     // Wave 2: MaxQueryMemory + QueryPlan-level warnings
@@ -370,6 +371,17 @@ public class PlanWarning
     public string Message { get; set; } = "";
     public PlanWarningSeverity Severity { get; set; }
     public SpillDetail? SpillDetails { get; set; }
+
+    /// <summary>
+    /// Maximum percentage of elapsed time that could be saved by addressing this finding.
+    /// null = not quantifiable, 0 = calculated as negligible.
+    /// </summary>
+    public double? MaxBenefitPercent { get; set; }
+
+    /// <summary>
+    /// Short actionable fix suggestion (e.g., "Add INCLUDE (columns) to index").
+    /// </summary>
+    public string? ActionableFix { get; set; }
 }
 
 public enum PlanWarningSeverity { Info, Warning, Critical }
@@ -433,6 +445,13 @@ public class PlanParameter
     public string? RuntimeValue { get; set; }
 }
 
+public class WaitBenefit
+{
+    public string WaitType { get; set; } = "";
+    public double MaxBenefitPercent { get; set; }
+    public string Category { get; set; } = "";
+}
+
 public class WaitStatInfo
 {
     public string WaitType { get; set; } = "";