fix(dross): collapse shell to 2 columns; topbar ◆ summons Dross

Removing #rightrail left a dead 360px grid column that narrowed #main.
Shell grid is now sidebar+main; the topbar ◆ (was Toggle-companion-rail)
now dispatches dross-toggle to open/close the floating bubble. Remaining
.rail-* CSS + chrome.js toggleRail are dead no-ops (minor cleanup later).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/components/dross_bubble.js

@@ -57,6 +57,8 @@ export async function renderDrossBubble() {
   fab.addEventListener('click', () => { if (fab._moved) { fab._moved = false; return; } openPanel(); });
   closeBtn.addEventListener('click', closePanel);
   collapse.addEventListener('click', closePanel);
+  // Topbar ◆ button (and any caller) can summon/dismiss Dross.
+  window.addEventListener('dross-toggle', () => panel.classList.contains('open') ? closePanel() : openPanel());
 
   drag(fab, fab, true); drag(header, panel, false);