Track DESIGN.md section 13 accepted risks and undocumented-behavior dependencies

## Problem / goal

DESIGN.md §13 names five accepted risks / open questions that Bastion knowingly takes on because the alternative requires banned undocumented APIs. Four of them (SW_MINIMIZE UX ceiling, desktop-switch inference, UIPI elevated classification, ApplicationFrameHost structure) are not "work to do" so much as ongoing risks to monitor via their Tier-5 canaries — this issue is the single place tracking their status so a future contributor doesn't have to reconstruct "why does Bastion do X instead of the obviously-simpler Y" from scratch. (Item 1, the cross-process corner-preference cosmetic flag, is concrete unimplemented work and has its own issue.)

Not scheduled to a milestone — this is a living tracking issue, not a unit of work with a finish line.

## References

- `DESIGN.md` §13 (all five items, verbatim).
- `docs/engineering/testing.md` §7 (Tier 5 canaries as the early-warning mechanism for all of these).

## Acceptance criteria

- [ ] Each of DESIGN.md §13 items 2-5 has a corresponding Tier-5 canary test once the feature it depends on ships (cross-reference from this issue as they land: item 3/4 from the workspace and elevated-window-detection issues, item 5 from the Window Registry issue's UWP attribution work).
- [ ] This issue's description is kept current if DESIGN.md §13 is ever revised (item added/resolved/superseded) — it is a living index, not a one-time snapshot.
- [ ] This issue is never closed as "completed" in the normal sense (there's no finish line for an accepted risk) — it's closed only if DESIGN.md §13 itself is restructured such that a single tracking issue no longer makes sense (e.g., split per-item).

Read DESIGN.md §13 and docs/engineering/testing.md §7 first, then compare items 2–5 with the current Tier-5 canary coverage and linked workspace, elevated-window-detection, and Window Registry work. Done means keeping this index and its cross-references current as those features and canaries land; it has no normal completion point.

Written by the indexing model from the issue text.