feat(panels): render collapse, dock and pin buttons from the store

PanelChrome no longer writes the panels' collapse buttons, the command
dock tray toggles' names or the dock pins' pressed state. It presents each
panel's open and pinned state to the new gev-panel-chrome store, and the
buttons, now views, call it there: PanelCollapseButton for the panel
headers, bound toggles and pins in the command dock. Clicks take effect
once the chrome is initialized; a new owner starts from its own states.

bindPanelDisclosure binds only a panel's Escape and refuses the retired
buttons and onChange arguments. UiButton joins the component kit.

src/panelStackLayout.test.mjs

@@ -167,7 +167,9 @@ test('desktop panel lanes use per-panel allocations and presentation-only auto-c
   );
   assert.match(ui, /panelId === 'radio-panel'[\s\S]*?document\.getElementById\('global-context-panel'\)/);
   assert.match(rightRail, /focusedExpandedPanel = expandedPanelsInDomOrder\.find\(\s*\(panel\) =>\s*panel\.contains\(documentRef\.activeElement\),?\s*\)/);
-  assert.match(ui, /setAttribute\('aria-expanded', String\(!collapsed\)\)/);
+  // Collapse buttons render aria-expanded from the state the chrome presents
+  // (panelChromeButtons.test.mjs covers the views).
+  assert.match(ui, /_chromeStore\.publish\(this\._chromeToken, panelEl\.id, \{\s*collapsed: panelEl\.classList\.contains\('collapsed'\),/);
   assert.match(leftRail, /--left-panel-allocated-height/);
   assert.match(rightRail, /--right-panel-allocated-height/);
   assert.match(