ECS stack post-merge review follow-ups

## Problem / Goal
Review items deferred from the ECS stack one-shot merge (#15544–#15786, landed via #15786) need tracked follow-up so no review comment is lost. 52 items across 9 theme sub-issues.

## Proposed Solution
One sub-issue per theme cluster; fix items per sub-issue checklist. Christian splits ownership via assignee field.

## Acceptance Criteria
- All 9 sub-issues closed with each checklist item fixed or explicitly waived.

Full context for agent readers

Pre-merge baseline: 92 unresolved review threads snapshotted 2026-08-25 20:03Z across the 7-PR stack. Reconciliation at tip e8129b14: 5 resolved on GitHub, 36 verified fixed-at-tip by re-review workers, 51 remaining → sub-issues, +1 new HIGH finding from combined-diff re-review. Merge plan: plans/stack-merge-plan-2026-08-25.md (workspace). Sub-issues unassigned by design.

Start by reviewing the nine theme sub-issues and their checklists, then read plans/stack-merge-plan-2026-08-25.md for the deferred review context. Done means all nine sub-issues are closed and every checklist item is fixed or explicitly waived.

Written by the indexing model from the issue text.