Plan analysis: PerformanceStudio's per-rule configuration, rule numbers, rule 38 and the legacy marker (#4535, #4566)

[erikdarlingdata]

Refs #4535 (the override mechanism is here; the configuration source that lets a user set overrides comes in the follow-up, so the issue stays open until then)
Fixes #4566
Refs #4530 (the entry points that pass the server metadata come in a follow-up)
Part of #4511

Why

PerformanceMonitor's and PerformanceStudio's plan analyzers are kept equivalent. This change ports PerformanceStudio's per-rule configuration plumbing, the rule numbers it tags each finding with, its Standard Edition DOP-2 rule, and its legacy-finding marker.

What changes

• AnalyzerConfig (with RulesConfig, disabled rules and severity overrides) and ServerMetadata are new types the analyzer accepts. PlanAnalyzer.Analyze and PlanAnalysisPipeline.Run gain overloads that take them; the existing no-argument overloads are unchanged and behave exactly as before (both pinned byte-for-byte identical).
• Every PlanWarning the analyzer constructs now carries a RuleNumber. A rule number census over the analyzer's source confirms this for every construction site, with two documented exceptions: rules 7 (spill severity) and 29 (implicit-conversion severity escalation) mutate an existing finding's severity/message rather than constructing a new one, so they carry a disable guard with no stamp.
• Each rule's !cfg.IsRuleDisabled(N) guard is wired at its own construction point, so a disabled rule produces no finding. A cross-step test disables every stamped rule number and confirms the fixtures then produce zero analyzer findings (SQL Server's own findings, which aren't numbered, are unaffected).
• A severity-override pass (ApplySeverityOverrides) applies configured severity overrides to any stamped, analyzer-sourced finding, walking into stored-procedure and UDF bodies the same way the analyzer's own statement walk does. SQL Server-sourced findings are never overridden.
• Rule 38 (Standard Edition DOP-2 limitation with batch mode) is new, as in PerformanceStudio dev: it looks at a statement that ran at DOP 2 with at least one batch-mode operator, unless the statement carries a MAXDOP 2 hint (masked for comments and literals, like rule 39's hint check), which makes the cap intentional. With the edition known to be Standard and the server's MAXDOP above 2 it is a Warning; with the edition unknown it is reported as a possible limitation (Info). Until the entry points pass server metadata (the follow-up), the edition is always unknown, so every rule-38 finding is Info.
• Findings of a known-legacy type (an internal list of 21 warning types, matching PerformanceStudio's) are marked IsLegacy = true on the finding, except when the finding came from SQL Server itself rather than the analyzer. This reaches the MCP plan tools' JSON (is_legacy per warning) and the plan viewer's warning header (an appended " [legacy]" tag, after the source tag).

Not yet

Loading a per-rule configuration from a file is the next PR. Until then, the default configuration applies everywhere: no rule is disabled, and no severity is overridden.

Test plan

Each step's own pins and REDs (recorded in the pre-merge step reports) plus, at the combine:

• The stale mid-rollout pin removed: PlanSync4535ConfigPlumbingTests.RuleNumber_DefaultsToNull_UntilLaterStepsStampIt pinned the state partway through the rollout (only rule 3 stamped) and is wrong now that every rule-stamping step has landed.
• New PlanSync4535RuleNumberCensusTests (replaces it): (a) a code census walking every new PlanWarning construction in PlanAnalyzer.cs, asserting each sets RuleNumber unless it's SQL-Server-sourced; (b) a runtime census over plan fixtures, asserting every analyzer-sourced finding (excluding the benefit scorer's Wait: findings, which aren't a numbered rule) has a non-null RuleNumber; (c) disabling every rule number the code census found, plus rules 7 and 29 (guard-only, no construction site), and confirming the fixtures then produce zero analyzer findings.
  • RED confirmed on the pre-merge sha (b20e89003): all 4 pins failed there (2 runtime failures from unstamped/undisabled rules, 1 empty-collection failure from having no rules to disable yet, and a missing-substring failure since the rule 7/29 guards didn't exist at that point).
  • Mutation: removed the RuleNumber = 20 stamp from the Local Variables construction site, rebuilt, confirmed the code census caught it (EveryAnalyzerConstructedWarning_StampsRuleNumber_UnlessSqlServerSourced [FAIL]), then reverted and rebuilt clean.
• Build: Darling.Tests, PerformanceMonitor.Ui, Lite/PerformanceMonitorLite.csproj, Lite.Tests, Darling/PerformanceMonitor.Darling.Service and deprecated/Dashboard.Tests all build Release with EnableWindowsTargeting=true: 0 warnings, 0 errors on every one of them.
• Run: in-process on macOS (Microsoft.WindowsDesktop.App stripped from the runtime config), every PlanSync* class plus BenefitScorerWiringTests, McpToolsListBudgetTests, McpPayloadContractCensusTests and the required non-PlanSync classes (ShowPlanParserCondAndMultiplePlanTests, the ActualPlan* classes, QueryModificationDetectorTests, ReproScriptBuilderHardeningTests, DarlingAnalysisPipelineTests, DarlingMcpPlanToolsSurfaceAndSqlTests, DarlingMcpPlanToolsLivePostgresTests, McpPlanAnalysisEnvelopeTests, SerialLoopStoreSizeSourceTests, TsqlConventionGuardTests, DocCommentHygieneTests):
  Total: 563, Errors: 0, Failed: 0, Skipped: 2, Not Run: 0. The 2 skips are the two live-Postgres tests that need DARLING_TEST_PG, unaffected by this change.
• Lite.Tests was built (0 warnings/0 errors) but not run — it targets net10.0-windows and can't run on macOS; CI decides it.

CHANGELOG

SECTION: Added
ENTRY: - Plan analysis flags a possible Standard Edition batch-mode DOP limit, and marks legacy findings ([#4585]) - A plan that ran at DOP 2 with batch-mode operators gets a Standard Edition DOP Limitation finding (Info until the server's edition is known). Findings from rules that predate the benefit-scoring framework carry is_legacy in the MCP plan output and a [legacy] tag in the plan viewer.
REF: [#4585]: #4585