web: world controls, slice 6 of M3

The lower grid completes: reach chart left, World card right. Two
headline levers (chance to forward, education budget as count plus
share) move instantly and rerun through the store's debounce; the
advanced disclosure holds students, friends per student, clique
tendency, first poster, and the three named seeds, each seed with a
dice reroll plus a Reroll world button (one coalesced run).

Engine 400s render inline under the card header in rose with the
offending field marked by name match; network failures keep using the
banner with retry. A graph-seed reroll remounts the network svg keyed
by graph hash, fading the new layout in over 200 ms.

web/src/components/ControlsCard.vue

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+<script setup lang="ts">
+import { computed } from 'vue'
+import AdvancedFields from './AdvancedFields.vue'
+import LeverSlider from './LeverSlider.vue'
+import { useSimStore } from '@/composables/useSimStore'
+import { formatPct } from '@/lib/format'
+
+// The world card (spec slice 6): two headline levers, the advanced grid,
+// and the inline spot for engine 400s. Lever changes hit the store
+// immediately and rerun every panel after the shared debounce.
+
+const store = useSimStore()
+
+const base = computed(() => store.state.base)
+
+const educationDisplay = computed(() => {
+  const share = (base.value.numEducated / Math.max(base.value.numStudents, 1)) * 100
+  return `${base.value.numEducated} · ${formatPct(share)}`
+})
+
+const offendsForwardProb = computed(
+  () => store.state.validationError?.includes('forwardProb') ?? false,
+)
+const offendsNumEducated = computed(
+  () => store.state.validationError?.includes('numEducated') ?? false,
+)
+</script>
+
+<template>
+  <section class="card ctl" aria-label="World settings">
+    <h3>World</h3>
+    <p v-if="store.state.validationError" class="invalid-note" role="alert">
+      {{ store.state.validationError }}
+    </p>
+    <LeverSlider
+      label="Chance to forward"
+      hint="per friendship, per round"
+      :value="base.forwardProb"
+      :min="0"
+      :max="1"
+      :step="0.01"
+      :display-value="formatPct(base.forwardProb * 100)"
+      :invalid="offendsForwardProb"
+      @update="(value) => store.setBaseField('forwardProb', value)"
+    />
+    <LeverSlider
+      label="Education budget"
+      hint="students the program reaches"
+      :value="base.numEducated"
+      :min="0"
+      :max="base.numStudents"
+      :step="1"
+      :display-value="educationDisplay"
+      :invalid="offendsNumEducated"
+      @update="(value) => store.setBaseField('numEducated', value)"
+    />
+    <AdvancedFields />
+  </section>
+</template>
+
+<style scoped>
+.card {
+  background: var(--surface);
+  border: 1px solid var(--border);
+  border-radius: var(--radius);
+  box-shadow: var(--shadow);
+  padding: 20px 22px;
+}
+
+h3 {
+  font-size: 13px;
+  font-weight: 600;
+  letter-spacing: 0.06em;
+  text-transform: uppercase;
+  color: var(--ink-4);
+}
+
+.invalid-note {
+  margin-top: 8px;
+  font-size: 12.5px;
+  font-weight: 500;
+  color: var(--spread);
+}
+</style>