sp_QuickieStore: add @find_high_impact parameter, fix PR #691 review items

@find_high_impact = 1 identifies the vital few queries consuming
disproportionate resources. Scores queries across 6 dimensions (CPU,
duration, reads, writes, memory, executions) using PERCENT_RANK,
shows share-of-total per metric, and surfaces diagnostic signals
(parameter sensitivity, plan instability, spills, wait time).
Tested on SQL Server 2016-2025.

Also fixes two items from Reece's tuning recommendations PR (#691):
- SUBSTRING parsing: added IIF fallback when CHARINDEX returns 0
  so field reordering in future SQL Server versions won't throw
  Msg 536 (invalid length parameter)
- OUTER APPLY for plan_force_recommendation_status: pulled into its
  own @new = 1 AND @expert_mode = 1 block so it doesn't run as
  wasted work when expert mode is off

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: erikdarlingdata