refactor(ui): drop the dead right rail policy and unused tab join hooks

shouldHideCollapsedRightPanels had become a constant: collapsed right-rail panels always hide behind the tab strip on desktop, so the function, its re-exports, the hud option of layoutRightPanelRail and the test of the constant go. The rail takes the theme's width as its content box instead of restating it per theme. Remove the unread --rail-tab-top property, dead CSS (an unset clip-path variable, an invisible Cyber highlight, redundant radius rules), an unused strip class, per-tab resize observation and guards that only served the test double, which now carries a style object.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

docs/CURRENT-STATE.md

@@ -1511,17 +1511,11 @@ the rail and the strip with it.
 
 The open tab and its panel are one surface, like a folder tab; the other tabs
 are borderless labels. The strip measures the open panel and tab, traces their
-joined silhouette as an SVG path (`railJoinPath`: rounded corners, concave
-fillets at the junction; Cyber's chamfered frame with its calibration bay and a
-square tab) and keeps a join element under the panels: one layer blurs and
-tints the backdrop inside the path, one SVG strokes the outline and another
-casts the drop shadow, so the junction has no seam or second blur region. The
-open panel's wrapper and card and the tab draw no surface of their own. The
-join is re-measured through a ResizeObserver and on strip scroll;
-`--rail-tab-bottom` on the rail sets the open panel's minimum height, so a
-short panel such as Context still reaches its tab. A first tab level with the
-panel's top shares its top edge; Cyber's strip starts below the frame's upper
-chamfer.
+joined silhouette as an SVG path (`railJoinPath`; Cyber gets its chamfered
+frame) and shapes a join element under the panels with it: glass, outline and
+shadow in one piece, so the junction has no seam. The panel and the tab draw
+no surface of their own. `--rail-tab-bottom` on the rail sets the open panel's
+minimum height, so a short panel such as Context still reaches its tab.
 
 A first-time visitor on a desktop viewport finds Display open
 (`src/ui/panelPositionControls.js`): with every panel closed nothing shows that