Add AnchorEdges [Flags] enum and Anchors property (default Left|Top, so
all existing elements are unchanged) to UiElement. ApplyAnchor() captures
the design-time margins on first call then recomputes Left/Top/Width/Height
each frame; DrawSelfAndChildren drives it for every child before painting.
ComputeAnchoredRect is public + static so it can be unit-tested without a
running frame loop.
MarkupDocument.Build gains a private Anchor() CSV parser and threads it
into the <meter> initializer via the anchor= attribute.
vitals.xml: remove title="Vitals" (retail vitals has no heading) and add
anchor="left,top,right" to all three meter bars so they stretch when the
panel is dragged wider.
Two new xUnit tests in UiRootInputTests: Left+Right stretches width;
Left+Top only keeps fixed size. All 19 App.Tests green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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