Ease the context popup's movement when it repositions

[Azganoth]

Summary

The context popup jumps between positions. Once it repositions against a changing selection, each change moves it a whole step at a time, and while a key repeat holds it moves once per keystroke. Easing that movement would make the popup read as one object following the text rather than a series of placements.

Expected behavior

The popup eases between positions when the selection it anchors to changes, including the repeated steps of a held Shift+Arrow.

Must remain stable: following the document scroll stays immediate, with no lag between the popup and the text it acts on; a popup opening does not animate in from its parked position; and a popup holding focus stays where it is.

Related context

• Related issues or pull requests: depends on Context popup keeps its position when the selection it anchors to changes #171, since there is nothing to ease until the popup repositions against its selection. Keyboard selection does not open the context popup that the documents promise #164 raised the motion as a separate presentation question and left it out.
• Documentation, decisions, or prior investigation: the popup's styling lives in MilkdownEditor.css under .leafdown-context-popup; the transform this would ease belongs to the Radix-owned wrapper around it.

Done when

• Selection-driven repositioning is eased, including under key repeat.
• Scrolling still moves the popup with no perceptible lag.
• Opening the popup does not animate it in from off-screen.
• Manual verification of all three; automated DOM tests cannot observe the motion.

Notes

Implementation direction

Radix positions through a transform on [data-radix-popper-content-wrapper], computed from React state, so scroll updates and selection updates are indistinguishable in CSS. Easing both would make the popup lag the text during a scroll, which is the one case that must stay immediate. What can tell them apart is the request: a selection change delivers a new one, a scroll does not, and EditorContextPopup.tsx already tracks that in a layout effect. Enabling the transition only while a new request settles is the smallest version.

PopperContent parks the wrapper at translate(0, -200%) until it is first positioned, so the transition must also be inactive for that first placement or the popup slides in from above on open.

Leafdown has no prefers-reduced-motion handling anywhere today. Whether this motion respects it, and whether that answer should apply to the app's existing open and close animations, is a decision this issue should settle rather than assume.

Out of scope

• Making the popup reposition in the first place, which is Context popup keeps its position when the selection it anchors to changes #171.
• The popup's open and close animations.