tui: add Tree component (collapsible device/POU listing)

<Tree project cursorPath expanded/> renders the project as a
two-level collapsible list: device headers with POU count, and (when
expanded) POU rows with name + kind + LOC count. Cursor row marked
with a leading triangle.

devicePath()/pouPath() are exported keypath helpers used by the tree
state machine to address rows uniformly.

src/tui/browser/Tree.tsx

@@ -0,0 +1,44 @@
+import React from 'react';
+import { Box, Text } from 'ink';
+import { Project, POU } from '../shared/types.js';
+
+export interface TreeProps {
+  project: Project;
+  cursorPath: string;
+  expanded: Set<string>;
+}
+
+export function devicePath(deviceName: string): string {
+  return `device:${deviceName}`;
+}
+export function pouPath(deviceName: string, relPath: string): string {
+  return `pou:${deviceName}:${relPath}`;
+}
+
+export function Tree({ project, cursorPath, expanded }: TreeProps): React.ReactElement {
+  const rows: React.ReactElement[] = [];
+  for (const dev of project.devices) {
+    const dPath = devicePath(dev.name);
+    const isExpanded = expanded.has(dPath);
+    const isCursor = cursorPath === dPath;
+    rows.push(
+      <Text key={dPath}>
+        {isCursor ? '▶ ' : '  '}
+        {isExpanded ? '▾ ' : '▸ '}
+        {dev.name}  {dev.pous.length} POUs
+      </Text>
+    );
+    if (!isExpanded) continue;
+    for (const p of dev.pous) {
+      const pPath = pouPath(dev.name, p.relPath);
+      const isPCursor = cursorPath === pPath;
+      rows.push(
+        <Text key={pPath}>
+          {isPCursor ? '▶ ' : '    '}
+          {p.name.padEnd(18)} {p.kind.padEnd(6)} {String(p.loc).padStart(4)} L
+        </Text>
+      );
+    }
+  }
+  return <Box flexDirection="column">{rows}</Box>;
+}

tests/tui/Tree.test.tsx

@@ -0,0 +1,58 @@
+import React from 'react';
+import { describe, it, expect } from 'vitest';
+import { render } from 'ink-testing-library';
+import { Tree } from '../../src/tui/browser/Tree.tsx';
+import { Project } from '../../src/tui/shared/types.ts';
+
+const project: Project = {
+  rootDir: '/p',
+  mirrorMtimeMs: 0,
+  devices: [
+    {
+      name: 'CodesysRpi',
+      pous: [
+        { name: 'PLC_PRG', kind: 'PRG', relPath: 'a/PLC_PRG.st', absPath: '/abs/a/PLC_PRG.st', loc: 5, mtimeMs: 0 },
+        { name: 'FB_Test', kind: 'FB',  relPath: 'a/FB_Test.st', absPath: '/abs/a/FB_Test.st', loc: 87, mtimeMs: 0 },
+      ],
+    },
+  ],
+};
+
+describe('<Tree>', () => {
+  it('renders devices and POU rows when expanded', () => {
+    const { lastFrame } = render(
+      <Tree
+        project={project}
+        cursorPath="device:CodesysRpi"
+        expanded={new Set(['device:CodesysRpi'])}
+      />
+    );
+    const out = lastFrame()!;
+    expect(out).toContain('CodesysRpi');
+    expect(out).toContain('PLC_PRG');
+    expect(out).toContain('FB_Test');
+    expect(out).toContain('PRG');
+    expect(out).toContain('FB');
+  });
+
+  it('does not show POU rows when device is collapsed', () => {
+    const { lastFrame } = render(
+      <Tree project={project} cursorPath="device:CodesysRpi" expanded={new Set()} />
+    );
+    const out = lastFrame()!;
+    expect(out).toContain('CodesysRpi');
+    expect(out).not.toContain('PLC_PRG');
+  });
+
+  it('marks the cursor row', () => {
+    const { lastFrame } = render(
+      <Tree
+        project={project}
+        cursorPath="pou:CodesysRpi:a/FB_Test.st"
+        expanded={new Set(['device:CodesysRpi'])}
+      />
+    );
+    const out = lastFrame()!;
+    expect(out).toMatch(/▶ FB_Test/);
+  });
+});