feat(reporting): loading states for scenario switch + filter changes

The scenario overlay uses keepPreviousData, so switching scenario or changing
a filter left the previous figures on screen with nothing signalling they were
being recalculated. Now:

- The picker row shows a small spinner + "Updating…" beside the filter chips
  while the overlay refetches — immediate feedback right at the controls. It
  reads react-query's useIsFetching(["scenario-report"]) so the query stays
  owned by MetricsBody (no lifting).
- The figures (KPI band + EPC distribution + ledger) dim (opacity-50, aria-busy)
  while refetching with stale data on screen, and show a spinner + "Updating
  figures…" / "Loading scenario…" line above them.

Verified end-to-end with a headless browser on portfolio 796: switching to an
"Increasing EPC" scenario and toggling a filter both surface the spinner,
"Updating…"/"Loading…" text, aria-busy and the dimmed figures.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app/portfolio/[slug]/(portfolio)/reporting/ReportingClientArea.tsx

@@ -1,19 +1,26 @@
 "use client";
 
 import { Suspense, use, useState } from "react";
-import { useQuery } from "@tanstack/react-query";
+import { useQuery, useIsFetching } from "@tanstack/react-query";
+import { ArrowPathIcon } from "@heroicons/react/24/outline";
 import { formatNumber, sapToEpc } from "@/app/utils";
 import {
   selectGoalCallout,
   shapeKpiDelta,
   toBandCounts,
 } from "@/lib/reporting/model";
-import { ScenarioCombobox, type ReportView } from "./components/ScenarioCombobox";
+import {
+  ScenarioCombobox,
+  type ReportView,
+} from "./components/ScenarioCombobox";
 import { KpiBand, type Kpi } from "./components/KpiBand";
 import { EpcChip, Panel } from "./components/primitives";
 import { EpcLadder } from "./components/EpcLadder";
 import { GoalCallout } from "./components/GoalCallout";
-import { InvestmentLedger, type LedgerView } from "./components/InvestmentLedger";
+import {
+  InvestmentLedger,
+  type LedgerView,
+} from "./components/InvestmentLedger";
 import { FilterChips, type ScenarioFilters } from "./components/FilterChips";
 import { DrillDownShelf, type DrillTarget } from "./components/DrillDownShelf";
 import { StockBreakdown } from "./components/StockBreakdown";
@@ -164,6 +171,12 @@ function ScenarioRow({
     typeof view === "number" ? scenarios.find((s) => s.id === view) : undefined;
   const showEpcFilters = selectedScenario?.goal === "Increasing EPC";
 
+  // Subscribe to the scenario overlay fetch (owned by MetricsBody) without
+  // lifting it — so applying a filter shows immediate feedback right here at
+  // the controls, not only down in the figures.
+  const fetchingCount = useIsFetching({ queryKey: ["scenario-report"] });
+  const busy = isScenario && fetchingCount > 0;
+
   return (
     <div className="flex flex-wrap items-center gap-3">
       <ScenarioCombobox scenarios={scenarios} value={view} onChange={onView} />
@@ -183,7 +196,9 @@ function ScenarioRow({
             {selectedScenario.goal === "Increasing EPC" && (
               <>
                 Target{" "}
-                <b className="text-gray-900">EPC {selectedScenario.goalValue}</b>{" "}
+                <b className="text-gray-900">
+                  EPC {selectedScenario.goalValue}
+                </b>{" "}
                 ·{" "}
               </>
             )}
@@ -199,7 +214,17 @@ function ScenarioRow({
         )
       )}
       {isScenario && (
-        <div className="ml-auto">
+        <div className="ml-auto flex items-center gap-2">
+          {busy && (
+            <span
+              className="flex items-center gap-1 text-[0.72rem] text-gray-500"
+              role="status"
+              aria-live="polite"
+            >
+              <ArrowPathIcon className="h-3.5 w-3.5 animate-spin text-gray-400" />
+              Updating…
+            </span>
+          )}
           <FilterChips
             filters={filters}
             onChange={onFilters}
@@ -433,12 +458,12 @@ function MetricsBody({
   const ledger: LedgerView | null = scenarioData
     ? (() => {
         const carbonSaved =
-          (baseline.totals.total_carbon ?? 0) - Number(scenarioData.total_carbon);
+          (baseline.totals.total_carbon ?? 0) -
+          Number(scenarioData.total_carbon);
         const billSaved =
           (baseline.totals.total_bills ?? 0) - Number(scenarioData.total_bills);
         const n = scenarioData.n_units_upgraded || 1;
-        const capital =
-          scenarioData.construction_cost + scenarioData.pc_cost;
+        const capital = scenarioData.construction_cost + scenarioData.pc_cost;
         return {
           constructionCost: scenarioData.construction_cost,
           projectDelivery: scenarioData.pc_cost,
@@ -459,65 +484,85 @@ function MetricsBody({
       })()
     : null;
 
+  const scenarioBusy = isScenario && isFetching;
+
   return (
     <div className="space-y-4">
-      {/* KPI band */}
-      <KpiBand kpis={isScenario ? scenarioKpis : currentStockKpis} />
-      {isScenario && isFetching && !scenarioData && (
-        <p className="text-sm text-gray-600">Loading scenario…</p>
+      {scenarioBusy && (
+        <div
+          className="flex items-center gap-1.5 text-[0.8rem] text-gray-600"
+          role="status"
+          aria-live="polite"
+        >
+          <ArrowPathIcon className="h-3.5 w-3.5 animate-spin text-gray-400" />
+          {scenarioData ? "Updating figures…" : "Loading scenario…"}
+        </div>
       )}
 
-      {/* Charts + ledger */}
-      <div className="grid grid-cols-1 items-stretch gap-4 lg:grid-cols-[1.55fr_1fr]">
-        <Panel
-          title="EPC distribution"
-          meta={
-            isScenario
-              ? "Current vs scenario · click a band for its homes"
-              : "Effective performance · click a band for its homes"
-          }
-        >
-          <div className="mb-4">
-            <EpcLadder
-              bands={baseline.epcBands}
-              scenarioBands={scenarioBands}
-              selectedBand={drill?.filter === "band" ? drill.band : null}
-              onSelectBand={(band) =>
-                openDrill({
-                  filter: "band",
-                  band,
-                  chipBand: band,
-                  title: `Band ${band}`,
-                })
+      {/* Scenario figures dim while the overlay refetches — keepPreviousData
+          leaves the old numbers on screen, so this signals they're being
+          recalculated rather than letting them look silently up to date. */}
+      <div
+        className={`space-y-4 transition-opacity duration-200 ${
+          scenarioBusy && scenarioData ? "opacity-50" : ""
+        }`}
+        aria-busy={scenarioBusy}
+      >
+        {/* KPI band */}
+        <KpiBand kpis={isScenario ? scenarioKpis : currentStockKpis} />
+
+        {/* Charts + ledger */}
+        <div className="grid grid-cols-1 items-stretch gap-4 lg:grid-cols-[1.55fr_1fr]">
+          <Panel
+            title="EPC distribution"
+            meta={
+              isScenario
+                ? "Current vs scenario · click a band for its homes"
+                : "Effective performance · click a band for its homes"
+            }
+          >
+            <div className="mb-4">
+              <EpcLadder
+                bands={baseline.epcBands}
+                scenarioBands={scenarioBands}
+                selectedBand={drill?.filter === "band" ? drill.band : null}
+                onSelectBand={(band) =>
+                  openDrill({
+                    filter: "band",
+                    band,
+                    chipBand: band,
+                    title: `Band ${band}`,
+                  })
+                }
+              />
+            </div>
+            <GoalCallout
+              callout={callout}
+              total={total}
+              dimensionTotals={{ carbon: baseline.totals.total_carbon ?? 0 }}
+              onDrill={
+                callout.kind === "below-band"
+                  ? () =>
+                      openDrill({
+                        filter: "band",
+                        band: callout.band,
+                        chipBand: callout.band,
+                        title: `Below EPC ${callout.band}`,
+                      })
+                  : undefined
               }
             />
-          </div>
-          <GoalCallout
-            callout={callout}
-            total={total}
-            dimensionTotals={{ carbon: baseline.totals.total_carbon ?? 0 }}
-            onDrill={
-              callout.kind === "below-band"
-                ? () =>
-                    openDrill({
-                      filter: "band",
-                      band: callout.band,
-                      chipBand: callout.band,
-                      title: `Below EPC ${callout.band}`,
-                    })
-                : undefined
-            }
-          />
-        </Panel>
+          </Panel>
 
-        {isScenario && ledger ? (
-          <InvestmentLedger v={ledger} />
-        ) : (
-          <StockBreakdown
-            propertyTypes={propertyTypes}
-            ageBands={baseline.ageBands}
-          />
-        )}
+          {isScenario && ledger ? (
+            <InvestmentLedger v={ledger} />
+          ) : (
+            <StockBreakdown
+              propertyTypes={propertyTypes}
+              ageBands={baseline.ageBands}
+            />
+          )}
+        </div>
       </div>
 
       {/* Where the money goes (scenario only) */}