Port PerformanceStudio's legacy-rule marker: IsLegacy on warnings from pre-framework rules, shown as [legacy] #4566

Part of #4511, the sync of plan analysis from PerformanceStudio (PS) to PerformanceMonitor (PM).

erikdarlingdata/PerformanceStudio@aabbaa2 (erikdarlingdata/PerformanceStudio#266) did three things. It dropped rule 21, which #4537 ports, and it stripped the drafted wait-stat text, which PM already matches through the curated Resources/WaitStats.json. The third change is a legacy-rule marker, which PM doesn't have:

• PlanWarning.IsLegacy, exposed as is_legacy in PS's JSON result.
• A LegacyWarningTypes set (PS dev PlanAnalyzer.cs ~132) lists the warning types from rules that predate PS's benefit-scoring framework ([FEATURE] Add system to assign a "maximum benefit" for plan analysis rules PerformanceStudio#215): memory grants, local variables, index suggestions, table variables, scalar UDFs and others.
• MarkLegacyWarnings (PS dev PlanAnalyzer.Helpers.cs ~20–40) sets the flag on analyzer warnings of those types, and never on SQL Server's own warnings (Source == SqlServer). Analyze calls it per statement (PS dev PlanAnalyzer.cs ~116).
• The renderers show a " [legacy]" tag after the warning type: the viewer's warning lists (PS dev PlanViewerControl.Properties.cs ~862, ~948, ~1024), the advice cards, and the text and HTML output.

PM has the counterpart surfaces: PlanWarning, the MCP plan output (McpPlanAnalysisFormatter), and the shared viewer's warning headers (PlanWarningDisplay). The port is the field, the set, the marking pass, MCP is_legacy, and the viewer tag. PM has no HTML or text exporter, so those halves have no counterpart.

At 08f3e3c4e. PS reference: its dev branch at 85492a1.