tui: add plain-text Viewer (highlighting deferred)

<Viewer pou text scrollTop visibleRows/> renders a POU's source as
line-numbered text (4-col padding) with a bold header
'<name>.st (<kind>, <loc> L)'. Falls back to a dim '(no POU
selected)' when pou or text is null.

No syntax highlighting in v0.1 — that's intentional and tracked as
deferred. Smoke build only; render behavior is exercised through the
browser-mode integration tests in subsequent tasks.

src/tui/browser/Viewer.tsx

@@ -0,0 +1,34 @@
+import React from 'react';
+import { Box, Text } from 'ink';
+import { POU } from '../shared/types.js';
+
+export interface ViewerProps {
+  pou: POU | null;
+  text: string | null;
+  scrollTop: number;
+  visibleRows: number;
+}
+
+export function Viewer({ pou, text, scrollTop, visibleRows }: ViewerProps): React.ReactElement {
+  if (!pou || text == null) {
+    return (
+      <Box flexDirection="column">
+        <Text dimColor>(no POU selected)</Text>
+      </Box>
+    );
+  }
+  const lines = text.split(/\r?\n/);
+  const slice = lines.slice(scrollTop, scrollTop + visibleRows);
+  return (
+    <Box flexDirection="column">
+      <Text bold>
+        {pou.name}.st  ({pou.kind}, {pou.loc} L)
+      </Text>
+      {slice.map((l, i) => (
+        <Text key={scrollTop + i}>
+          {String(scrollTop + i + 1).padStart(4, ' ')}  {l}
+        </Text>
+      ))}
+    </Box>
+  );
+}