feat(ui): render the imagery split dividers from a store

createImagerySplit keeps the divider's value, drag, keyboard, CSS position
and scene split, and publishes its line to the imagery split store.
ImagerySplitView, in the scene chrome, teleports each line to the body, so
recent imagery and the Bhote Koshi event no longer build divider DOM.

docs/VUE-MIGRATION.md

@@ -139,7 +139,11 @@ a feature supplying only its accent.
    phone shell (`PhoneTabBar` renders the tabs and open sheet the shell
    publishes to the `gev-display` store, teleporting the tab bar and sheet
    backdrop to the end of the page from the first publish until the shell
-   is destroyed; the shell takes their clicks from the document), then
+   is destroyed; the shell takes their clicks from the document), the
+   imagery dividers (`ImagerySplitView`, in the scene chrome, teleports each
+   before/after divider that `createImagerySplit` publishes to the
+   `gev-imagery-split` store to the body; the split keeps the
+   drag, keyboard, CSS position and scene split), then
    Cockpit (in progress: `CockpitBriefing` renders the briefing card's
    pages, cycle toggle, regional news and local conditions, and
    `CockpitContext` the Contact card's scope, navigation, counts, nearest