feat(family): platforms declared at inception and detected from bound repos (milestone 463 step 2, #4988)

A project's platforms decide which family ideas reach it. This step makes membership answerable from every door:

- services/platforms.py: the global catalog (writes are admin-only and duplicate-gated by slug); pure marker detection; and membership reads and writes. Detection only ADDS, and only where nobody has answered. It never overrides a declared or rejected row and never removes one.
- coverage: the archive scan now carries every path, and the refresh runs detection fail-open.
- inception: a platforms choice (slugs, or null for unanswered). The list is the whole answer: members left out of it become rejected.
- MCP: list_platforms and set_project_platforms; enter_project and get_project carry the project's platforms.
- REST: /api/platforms (admin writes) and /api/projects/<id>/platforms.
- UI: a platforms checklist on the inception card, a Family tab on ProjectView, and a Platforms admin tab in Settings.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/components/InceptionCard.vue

@@ -6,10 +6,16 @@
  * second step and only emits the choices (the project does not exist yet);
  * mode="decide" sits on ProjectView for an undecided project, loads that
  * project's current defaults, and records the decision itself.
+ *
+ * Platforms (milestone 463) start UNANSWERED (null). Ticking any box makes
+ * the list the whole answer; in decide mode the boxes start from what
+ * detection already found, so recording confirms it.
  */
 import { computed, onMounted, ref, watch } from "vue";
 import { apiErrorMessage } from "@/api/client";
 import { fetchDesignSystems } from "@/api/designSystems";
+import { MEMBER_STATES } from "@/api/platforms";
+import { usePlatformsStore } from "@/stores/platforms";
 import {
   decideInception, emptyChoices, fetchInceptionDefaults,
   type InceptionChoices, type InceptionDecision, type InceptionDefaults,
@@ -29,6 +35,9 @@ const emit = defineEmits<{
 const local = ref<InceptionChoices>(props.choices ? { ...props.choices } : emptyChoices());
 const designSystems = ref<{ id: number; title: string }[]>([]);
 const systemsCount = ref(0);
+const platforms = ref<{ slug: string; name: string }[]>([]);
+const detected = ref<Set<string>>(new Set());
+const platformsStore = usePlatformsStore();
 const loading = ref(true);
 const saving = ref(false);
 const error = ref("");
@@ -46,11 +55,25 @@ async function load() {
       const d: InceptionDefaults = await fetchInceptionDefaults(props.projectId);
       designSystems.value = d.design_systems;
       systemsCount.value = d.systems;
+      platforms.value = d.platforms;
+      const members = d.project_platforms
+        .filter((p) => MEMBER_STATES.includes(p.state))
+        .map((p) => p.slug);
+      detected.value = new Set(
+        d.project_platforms.filter((p) => p.state === "detected").map((p) => p.slug),
+      );
       // Start from what stands today, so "record" without changes keeps it.
-      local.value = { design_system_id: d.design_system_id, seed_systems: false };
+      local.value = {
+        design_system_id: d.design_system_id,
+        seed_systems: false,
+        platforms: members.length ? members : null,
+      };
     } else {
-      const ds = await fetchDesignSystems();
+      const [ds, catalog] = await Promise.all([
+        fetchDesignSystems(), platformsStore.fetchCatalog(),
+      ]);
       designSystems.value = ds.design_systems.map((d) => ({ id: d.id, title: d.title }));
+      platforms.value = catalog.map((p) => ({ slug: p.slug, name: p.name }));
     }
   } catch (e: unknown) {
     error.value = apiErrorMessage(e, "Could not load what this project could inherit");
@@ -61,6 +84,22 @@ async function load() {
 
 const nothingToDecide = computed(() => !designSystems.value.length);
 
+function isChecked(slug: string): boolean {
+  return (local.value.platforms ?? []).includes(slug);
+}
+
+function togglePlatform(slug: string, on: boolean) {
+  const current = new Set(local.value.platforms ?? []);
+  if (on) current.add(slug);
+  else current.delete(slug);
+  local.value.platforms = [...current].sort();
+}
+
+/** Back to "not answered" — distinct from an empty list, which says "none". */
+function clearPlatforms() {
+  local.value.platforms = null;
+}
+
 async function record() {
   if (!props.projectId) return;
   saving.value = true;
@@ -105,6 +144,31 @@ onMounted(load);
           </span>
         </label>
       </div>
+      <div v-if="platforms.length" class="inception-group">
+        <h4>Platforms</h4>
+        <p class="inception-muted">
+          What it is built on or ships as. Ideas the family has proven for a
+          platform reach every project that is one.
+          <template v-if="local.platforms === null"> Not answered yet.</template>
+          <template v-else>
+            Anything left unticked is recorded as "not this project".
+            <button type="button" class="btn-ghost btn-sm" @click="clearPlatforms">Leave unanswered</button>
+          </template>
+        </p>
+        <div class="inception-platforms">
+          <label v-for="p in platforms" :key="p.slug" class="inception-choice">
+            <input
+              type="checkbox"
+              :checked="isChecked(p.slug)"
+              @change="togglePlatform(p.slug, ($event.target as HTMLInputElement).checked)"
+            />
+            <span>
+              {{ p.name }}
+              <em v-if="detected.has(p.slug)" class="inception-muted"> — detected</em>
+            </span>
+          </label>
+        </div>
+      </div>
       <p v-if="nothingToDecide" class="inception-muted">
         No design systems on this install yet — recording still settles the question.
       </p>
@@ -132,6 +196,11 @@ onMounted(load);
 .inception-group h4 { margin: 0 0 0.35rem; font-size: 0.9rem; font-weight: 500; }
 .inception-choice { display: flex; align-items: flex-start; gap: 0.5rem; font-size: 0.9rem; margin: 0.25rem 0; }
 .inception-choice input { margin-top: 0.2rem; accent-color: var(--fs-accent); }
+.inception-platforms {
+  display: grid;
+  grid-template-columns: repeat(auto-fill, minmax(12rem, 1fr));
+  gap: 0 1rem;
+}
 .inception-select {
   padding: 0.45rem 0.7rem;
   border: 1px solid var(--fs-border-color);