fix: put the recall chip in the header utilities, not the cell the sidebar's expand button owns

The chip registered into `conversation.session.header.corner`, which is a
SINGLE cell the product already occupies: the client catalog lists its
occupants as `client-ui-sidebar-right ExpandButton` and its replaceRisk as
`shadows-shipped-ui`. Two registrations at priority 0 in a single cell are a
hard error inside the slot registry, and the error lands in the OTHER
plugin's `slots.inject()` callback: the renderer's `changed` handler catches
it, calls `stop()` on that inject controller — killing that contribution for
the rest of the fiber's life — and rethrows it from a microtask, which is the
uncaught `single slot ... already has a registration at priority 0 (registered
by dsh-recall)` in the console. So the chip was not merely racing the shipped
right-sidebar expand control; it was taking it down.

It now registers into `conversation.session.header.utilities`, a list cell
(`replaceRisk: none`, right-aligned, same Session scope and therefore the same
`useProjection` prop) as `{ id: 'dsh-recall', order: 20 }`, so it stays at the
right-hand end without competing for a seat.

The suite could not have caught this: the slot stub accepts any registration
and knows nothing about cardinality, keys or collisions. It now asserts the
chip carries a list id and that this plugin registers nothing into the corner.

client/client.js

@@ -4,7 +4,7 @@
  * Three contributions, each into a seat the Harness 0.1.7 composition declares:
  *
  *   1. the plugin's configuration page, in the **Plugins** panel;
- *   2. a one-line usage chip in a conversation's header corner;
+ *   2. a one-line usage chip in a conversation's header utilities;
  *   3. a right-sidebar tab with the same figures and room for their caveats.
  *
  * **Where plugin settings live now.** 0.1.5 drew plugin configuration as cards
@@ -620,8 +620,8 @@ window.__ModuleLoader__.load({
 
     function RecallChip(props) {
       const usage = useUsage(props)
-      // The corner is laid out only while its occupant renders something, so a
-      // conversation that never called the tool costs no layout at all.
+      // The utilities list draws nothing for an entry that renders nothing, so
+      // a conversation that never called the tool costs no layout at all.
       if (usage === undefined || !usage.searches) return null
       return h(
         'span',
@@ -735,10 +735,28 @@ window.__ModuleLoader__.load({
         )
       })
 
-      // The chip: the header's far-right corner, which the framework lays out
-      // only while its occupant renders something.
-      ctx.slots.inject('conversation.session.header.corner', () =>
-        ctx.slots.register({ name: 'conversation.session.header.corner' }, (props) => h(RecallChip, props)),
+      // The chip: a right-aligned session utility, next to the header's
+      // far-right corner.
+      //
+      // It is in the `utilities` LIST, not the corner, and that is not a
+      // cosmetic choice. The corner is a `single` cell whose occupant the
+      // product already declares — `client-ui-sidebar-right ExpandButton`, the
+      // right sidebar's expand control — and the client catalog reports it as
+      // `replaceRisk: "shadows-shipped-ui"`. A second registration at the same
+      // priority is a hard error inside the slot registry, thrown while the
+      // other plugin's `slots.inject()` callback runs: the renderer catches it,
+      // STOPS that inject controller for the rest of the fiber's life, and
+      // rethrows it uncaught. So squatting there did not merely race — it took
+      // the shipped expand control down and logged an uncaught error. A list
+      // entry carrying its own id cannot collide with anyone, and the framework
+      // draws nothing for an entry that renders nothing.
+      ctx.slots.inject('conversation.session.header.utilities', () =>
+        ctx.slots.register(
+          // `order` ascends left to right, so a high value keeps the chip at
+          // the right-hand end of the utilities, where it was before.
+          { name: 'conversation.session.header.utilities', id: 'dsh-recall', order: 20 },
+          (props) => h(RecallChip, props),
+        ),
       )
 
       // The right-sidebar tab is registered in two stages, and the second is