No phone, so stop designing for one

Targets are a 12-inch tablet and a desktop (D15). That removes most of the
navigation question rather than answering it.

`EXPANDED_MIN_WIDTH` is 820 logical pixels and a 12-inch tablet is ~1024
across in portrait, so both orientations of both targets are the expanded
class. The compact class now fires only when a desktop window is dragged
narrow — graceful degradation, not a second interface. The bottom tool strip
and the one-tool-at-a-time sheet were solving a phone, and there is no phone.

What survives is input, not size, and the architecture had already decided
it: `WidgetDemand::precise_pointing` exists for a television remote, and its
own documentation says touch is fine because hit regions grow to the
modality. Touch changes hit regions, not layout. The rules that fall out are
worth stating because they are easy to violate by accident — no hover-only
affordance and no modifier key may be the sole route to anything, since a
tablet has neither. Local masking already lost its shift-click extend for
exactly this reason.

The guaranteed-wide viewport also pays for a better answer to the extent
problem than hiding things. The complaint was never that the column is long;
it is that the histogram scrolls away from the sliders it reports on.
Collapsing shortens the scroll, pinning removes the problem, and ~260px of
fixed height is affordable on a viewport that is never under 820 wide.

docs/architecture.md

@@ -1086,6 +1086,7 @@ Full rationale in [requirements.md §8](requirements.md). Summary:
 | D12 | Scope versus pace | **Open** |
 | D13 | Face inference runtime and model licensing | **Runtime answered**, licensing open |
 | D14 | Segmentation source for local masking | Decided — arm C (docs/segmentation.md §14) |
+| D15 | Target devices — 12-inch tablet and desktop, no phone | Decided (requirements D15) |
 
 ---