Harden against the full DESIGN.md section 9 edge-case matrix

## Problem / goal

Closing, cross-cutting hardening pass against the full DESIGN.md §9 edge-case matrix, ahead of v1.0. This is not a first implementation of any single row — most rows already have a dedicated implementation issue in an earlier milestone — it is the pass that verifies nothing regressed and nothing was missed.

## References

- `DESIGN.md` §9 — the full table: elevated windows, Snap adoption, hung apps, fullscreen games, Electron/Chromium quirks, UWP/ApplicationFrameHost, Explorer restart, event storms, self-moving apps, HWND recycling.

## Acceptance criteria

- [ ] Every row in the DESIGN.md §9 table has either a passing Tier 2/3 test exercising its documented behavior, or an explicitly filed follow-up issue explaining why it's deferred past v1.0.
- [ ] No row's "ladder floor" (Unmanage / Float / Quarantine / Pause monitor / etc.) silently regresses to a harder failure than documented.
- [ ] A pass through this issue produces a written summary (PR description or a doc under `docs/`) enumerating which rows were newly covered vs. already covered by earlier milestones.

Start with DESIGN.md §9 and compare every row with the existing Tier 2/3 coverage from earlier milestones. Record rows with passing tests and document deferred rows in a follow-up issue; verify that each ladder floor matches the documented behavior and provide the required coverage summary in the PR description or under docs/.

Written by the indexing model from the issue text.