Add Collapse atom for progressive disclosure

Thin MUI Collapse wrapper with unmountOnExit default. Used in the
arrangement wizard to reveal fields after a selection is made
(slide-down animation). Stories include interactive toggle and
wizard field-reveal pattern demo.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/atoms/Collapse/Collapse.stories.tsx

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+import { useState } from 'react';
+import type { Meta, StoryObj } from '@storybook/react';
+import { Collapse } from './Collapse';
+import Box from '@mui/material/Box';
+import { Typography } from '../Typography';
+import { Button } from '../Button';
+
+const meta: Meta<typeof Collapse> = {
+  title: 'Atoms/Collapse',
+  component: Collapse,
+  tags: ['autodocs'],
+  parameters: {
+    layout: 'centered',
+  },
+  argTypes: {
+    in: {
+      control: 'boolean',
+      description: 'Whether the content is expanded',
+    },
+    timeout: {
+      control: 'number',
+      description: 'Transition duration in ms (or { enter, exit })',
+      table: { defaultValue: { summary: '300' } },
+    },
+  },
+};
+
+export default meta;
+type Story = StoryObj<typeof Collapse>;
+
+// ─── Default (controlled via args) ──────────────────────────────────────────
+
+/** Toggle the `in` control to expand/collapse */
+export const Default: Story = {
+  args: {
+    in: true,
+  },
+  render: (args) => (
+    <Box sx={{ width: 400 }}>
+      <Collapse {...args}>
+        <Box
+          sx={{
+            p: 3,
+            bgcolor: 'var(--fa-color-brand-50)',
+            borderRadius: 1,
+            border: '1px solid',
+            borderColor: 'divider',
+          }}
+        >
+          <Typography variant="body1">
+            This content is revealed with a smooth slide-down animation.
+          </Typography>
+        </Box>
+      </Collapse>
+    </Box>
+  ),
+};
+
+// ─── Interactive toggle ─────────────────────────────────────────────────────
+
+/** Click the button to toggle progressive disclosure */
+export const Interactive: Story = {
+  render: () => {
+    const [open, setOpen] = useState(false);
+    return (
+      <Box sx={{ width: 400 }}>
+        <Button variant="soft" color="secondary" onClick={() => setOpen(!open)} sx={{ mb: 2 }}>
+          {open ? 'Hide details' : 'Show details'}
+        </Button>
+        <Collapse in={open}>
+          <Box
+            sx={{
+              p: 3,
+              bgcolor: 'var(--fa-color-brand-50)',
+              borderRadius: 1,
+              border: '1px solid',
+              borderColor: 'divider',
+            }}
+          >
+            <Typography variant="body1" sx={{ mb: 1 }}>
+              Additional details are revealed here.
+            </Typography>
+            <Typography variant="body2" color="text.secondary">
+              This pattern is used in the wizard for progressive disclosure — fields appear after a
+              previous selection is made.
+            </Typography>
+          </Box>
+        </Collapse>
+      </Box>
+    );
+  },
+};
+
+// ─── Wizard field reveal ────────────────────────────────────────────────────
+
+/** Simulates the wizard pattern: selecting an option reveals the next field */
+export const WizardFieldReveal: Story = {
+  render: () => {
+    const [step, setStep] = useState(0);
+    return (
+      <Box sx={{ width: 400 }}>
+        <Typography variant="label" sx={{ mb: 1, display: 'block' }}>
+          Who is this funeral being arranged for?
+        </Typography>
+        <Box sx={{ display: 'flex', gap: 2, mb: 2 }}>
+          <Button
+            variant={step >= 1 ? 'contained' : 'soft'}
+            color={step >= 1 ? 'primary' : 'secondary'}
+            size="large"
+            fullWidth
+            onClick={() => setStep(1)}
+          >
+            Myself
+          </Button>
+          <Button
+            variant={step >= 2 ? 'contained' : 'soft'}
+            color={step >= 2 ? 'primary' : 'secondary'}
+            size="large"
+            fullWidth
+            onClick={() => setStep(2)}
+          >
+            Someone else
+          </Button>
+        </Box>
+
+        <Collapse in={step >= 2}>
+          <Box sx={{ mt: 1 }}>
+            <Typography variant="label" sx={{ mb: 1, display: 'block' }}>
+              Has the person died?
+            </Typography>
+            <Box sx={{ display: 'flex', gap: 2 }}>
+              <Button variant="soft" color="secondary" size="large" fullWidth>
+                Yes
+              </Button>
+              <Button variant="soft" color="secondary" size="large" fullWidth>
+                No
+              </Button>
+            </Box>
+          </Box>
+        </Collapse>
+      </Box>
+    );
+  },
+};
+
+// ─── Collapsed ──────────────────────────────────────────────────────────────
+
+/** Content is hidden (collapsed state) */
+export const Collapsed: Story = {
+  args: {
+    in: false,
+  },
+  render: (args) => (
+    <Box sx={{ width: 400 }}>
+      <Typography variant="body2" color="text.secondary" sx={{ mb: 1 }}>
+        The content below is collapsed:
+      </Typography>
+      <Collapse {...args}>
+        <Box sx={{ p: 3, bgcolor: 'var(--fa-color-brand-50)', borderRadius: 1 }}>
+          <Typography variant="body1">You should not see this.</Typography>
+        </Box>
+      </Collapse>
+    </Box>
+  ),
+};

src/components/atoms/Collapse/Collapse.tsx

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+import React from 'react';
+import MuiCollapse from '@mui/material/Collapse';
+import type { CollapseProps as MuiCollapseProps } from '@mui/material/Collapse';
+
+// ─── Types ───────────────────────────────────────────────────────────────────
+
+/** Props for the FA Collapse component */
+export interface CollapseProps extends MuiCollapseProps {
+  /** Whether the content is expanded */
+  in: boolean;
+  /** Content to reveal/hide */
+  children: React.ReactNode;
+}
+
+// ─── Component ───────────────────────────────────────────────────────────────
+
+/**
+ * Progressive disclosure wrapper for the FA design system.
+ *
+ * Thin wrapper around MUI Collapse with sensible defaults for the
+ * arrangement wizard's progressive disclosure pattern (fields revealed
+ * after a selection is made).
+ *
+ * Uses a smooth slide-down animation. Unmounts children when collapsed
+ * to keep the DOM clean and prevent focus on hidden fields.
+ *
+ * Usage:
+ * ```tsx
+ * <Collapse in={hasSelectedOption}>
+ *   <FormField label="Next question" />
+ * </Collapse>
+ * ```
+ */
+export const Collapse = React.forwardRef<HTMLDivElement, CollapseProps>(
+  ({ children, ...props }, ref) => (
+    <MuiCollapse ref={ref} unmountOnExit {...props}>
+      {children}
+    </MuiCollapse>
+  ),
+);
+
+Collapse.displayName = 'Collapse';
+export default Collapse;

src/components/atoms/Collapse/index.ts

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+export { default } from './Collapse';
+export * from './Collapse';