A map's own ? can ask its host for help: HelpButton and useHostedHelp send doxa:help

The Experimental maps' explainer lived in a "?" floating at the top centre of the app, which read
as a game piece rather than part of the map. The map already has a "?" control. This lets that
control answer from the HOST: a click dispatches doxa:help, bubbling and composed, so it crosses
every shadow root between the button and the page, and the app answers with the map's text.

library/composables/useHostedHelp.js is the one mechanism, for the HelpButton map control and for
the maps whose "?" belongs in their own bar. It also decides WHERE the button shows: only under a
host that answers (an ancestor, across shadow roots, carrying data-doxa-help), so the same bundle
embedded on a partner page shows no dead "?". HelpButton keeps its open-help emit, so
doxa-simple-map's own help popup is unchanged; hosted is opt-in.

library/components/map-controls/HelpButton.vue

@@ -1,30 +1,47 @@
 <!--
-  HelpButton.vue — Opens the "About this map" help modal.
+  HelpButton.vue — the map's "?" control: what this map is and how to use it.
 
-  Service logic extension point: swap the `open-help` emit for an inline
-  tooltip, an external link, a Pinia help-store action, etc.
+  One click, two ways to answer it:
+    - `open-help` (emit): the map shows its own help (doxa-simple-map's popup does).
+    - `doxa:help` (a DOM event, bubbling and COMPOSED; see composables/useHostedHelp.js): a HOST
+      answers for the map. The Doxa Maps app (doxa-maps-spa) answers with the Experimental shelf's
+      what/how text, which is how that explainer left the top of the screen for the map's own control
+      column, next to zoom and fullscreen.
 
   Props:
     isDark — theme flag forwarded from parent
+    hosted — show the button ONLY under a host that answers doxa:help. A map whose help lives in its
+             host sets this, so the same bundle embedded on a page with no such host shows no dead "?".
 
   Emits:
     open-help — fires on click; parent decides how to display help content
 -->
 <script setup>
+import { computed } from 'vue'
 import { useI18n } from 'vue-i18n'
 import MapControlButton from './MapControlButton.vue'
+import { useHostedHelp } from '../../composables/useHostedHelp.js'
 
 const { t } = useI18n()
 
-defineProps({
-  isDark: { type: Boolean, default: false }
+const props = defineProps({
+  isDark: { type: Boolean, default: false },
+  hosted: { type: Boolean, default: false }
 })
 
-defineEmits(['open-help'])
+const emit = defineEmits(['open-help'])
+
+const { hosted: underHost, askForHelp } = useHostedHelp()
+const shown = computed(() => !props.hosted || underHost.value)
+
+function onClick() {
+  emit('open-help')
+  askForHelp()
+}
 </script>
 
 <template>
-  <MapControlButton data-control="help" :is-dark="isDark" :title="t('buttons.aboutThisMap')" @click="$emit('open-help')">
+  <MapControlButton v-if="shown" data-control="help" :is-dark="isDark" :title="t('buttons.aboutThisMap')" @click="onClick">
     <span style="font-size:15px;font-weight:700;font-family:serif;line-height:1;">?</span>
   </MapControlButton>
 </template>

library/composables/useHostedHelp.js

@@ -0,0 +1,41 @@
+/**
+ * useHostedHelp — a map's "?" that its HOST answers.
+ *
+ * A map embedded in the Doxa Maps app (doxa-maps-spa) does not carry its own explainer: the app
+ * holds the what/how text for every Experimental map. So the map's "?" does not open anything
+ * itself. It dispatches `doxa:help`, bubbling and COMPOSED, which crosses every shadow root between
+ * the button and the page, and the app answers with that map's text.
+ *
+ * WHERE THE BUTTON SHOWS. Only under a host that answers: an ancestor, across shadow roots, carrying
+ * `data-doxa-help` (the app sets it on the pane of every tab that has text). The same bundle
+ * embedded on a partner page has no such ancestor, so it shows no "?" rather than a dead one.
+ *
+ * Two callers: the library HelpButton (a map control) and the few maps whose "?" belongs in their
+ * own bar instead (the Wall, the vision card). One mechanism, so the two cannot drift apart.
+ *
+ *   const { hosted, askForHelp } = useHostedHelp()
+ *   <button v-if="hosted" @click="askForHelp($event.currentTarget)">?</button>
+ */
+import { getCurrentInstance, onMounted, ref } from 'vue'
+
+/** Is `node` inside a host that answers doxa:help? Walks up, stepping out of each shadow root. */
+export function underHelpHost(node) {
+  for (let n = node; n; n = n.parentNode || n.host) {
+    if (n.nodeType === 1 && n.hasAttribute('data-doxa-help')) return true
+  }
+  return false
+}
+
+/** Ask the host for this map's help, from `el` (the button, so the host can return focus to it). */
+export function askForHelp(el) {
+  if (el && el.dispatchEvent) el.dispatchEvent(new CustomEvent('doxa:help', { bubbles: true, composed: true }))
+}
+
+export function useHostedHelp() {
+  const self = getCurrentInstance()
+  const hosted = ref(false)
+  // Mounted means connected: a custom element mounts from connectedCallback, so the chain up to the
+  // host's pane is already in place. $el may be a comment or fragment anchor; it is in the tree too.
+  onMounted(() => { hosted.value = underHelpHost(self && self.proxy && self.proxy.$el) })
+  return { hosted, askForHelp: (el) => askForHelp(el || (self && self.proxy && self.proxy.$el)) }
+}