scroll-stress-test advances its randomized torture chamber

@tvondra spent the week hardening tests for scrollable cursors on index scans.

Six commits landed in scroll-stress-test this week, pushing forward the randomized stress testing apparatus for scrollable cursors during index scans. The work remains unpublished as a release, but the commit velocity suggests active refinement of the test harness itself. No major features or API changes announced, but the infrastructure work hints at preparation for either a release cycle or upstream integration.