Epic: DarlingData-inspired diagnostics and engineering improvements

[nanoDBA]

Summary

Adoption candidates distilled from an in-depth study of Erik Darling's DarlingData diagnostic procedures (sp_PressureDetector, sp_PerfCheck, sp_HealthParser, sp_LogHunter, sp_HumanEvents, sp_QueryReproBuilder, sp_IndexCleanup, sp_QuickieStore), plus concrete defects found by auditing this codebase against that study's gotcha checklist. Each sub-issue is pre-screened against existing functionality to avoid duplicates.

Sub-issues

Adoption candidates

• Proc: pressure gates + END-record pressure covariates (memory-grant semaphore, THREADPOOL warning, IO-stall + WAIT_ON_SYNC_STATISTICS_REFRESH deltas) #614 Proc: pressure gates + END-record pressure covariates
• Proc: debug-printable dynamic discovery SQL + phase breadcrumbs in CATCH output #615 Proc: debug-printable dynamic discovery SQL + phase breadcrumbs
• Diag: detect stats on disabled/hypothetical indexes and duplicate-leading-column statistics #616 Diag: stats on disabled/hypothetical indexes, duplicate-leading-column stats
• Diag: system_health correlation result set + error-log evidence for killed runs #617 Diag: system_health correlation + error-log evidence for killed runs
• XML performance + robustness pass: .exist()+sql:variable predicates, control-char scrub #618 XML performance + robustness pass
• Diag: repro-builder result set — reconstruct regressed queries after stats updates #619 Diag: repro-builder result set for regressed queries
• Generate @Help parameter documentation from sys.all_parameters (kills doc drift) #620 Generate @Help from sys.all_parameters
• XE session overhaul: event_file target, stale-session hygiene, Azure ON DATABASE, CONTEXT_INFO-scoped predicates #621 XE session overhaul

Audit findings (from applying the study's checklist to this codebase)

• Diag/proc: DATEDIFF(MILLISECOND) int overflow on CommandLog rows spanning >24.8 days hard-fails the diagnostic run #622 DATEDIFF(MILLISECOND) int overflow hard-fails Diag
• Diag: CpuChangePct has no denominator floor — tiny-baseline stats produce absurd percentages that dominate the trend headline #623 CpuChangePct denominator floor
• Diag: sub-check failures should surface as recommendation rows, not console-only messages #624 Sub-check failures as recommendation rows
• Diag: wrap long SQL text columns in processing-instruction XML for clickable SSMS output #625 processing-instruction wrapping for SQL text columns