tui(approve): v toggles unified <-> side-by-side diff

Side-by-side renders old | new in two 50% columns separated by '│'.
Consecutive del/add hunks are paired row-by-row; a longer side gets
blank rows on the shorter side. Context lines mirror on both sides.

Footer now lists 'v toggle side-by-side'.

src/tui/approve/Approve.tsx

@@ -16,8 +16,13 @@ export function Approve({ fileName, oldText, newText, onDecision }: ApproveProps
   const hunks = React.useMemo(() => computeHunks(oldText, newText), [oldText, newText]);
   const adds = hunks.filter((h) => h.kind === 'add').length;
   const dels = hunks.filter((h) => h.kind === 'del').length;
+  const [sideBySide, setSideBySide] = React.useState(false);
 
   useInput((input, key) => {
+    if (input === 'v') {
+      setSideBySide((v) => !v);
+      return;
+    }
     if (input === 'y') return onDecision('accept');
     if (input === 'n' || input === 'q' || key.escape) return onDecision('reject');
   });
@@ -27,18 +32,24 @@ export function Approve({ fileName, oldText, newText, onDecision }: ApproveProps
       <Text>
         ─ Approve change? {fileName} ─── + {adds} lines, − {dels} lines ─
       </Text>
-      <Box flexDirection="column">
-        {hunks.map((h, i) => (
-          <HunkLine key={i} hunk={h} />
-        ))}
-      </Box>
+      {sideBySide ? <SideBySide hunks={hunks} /> : <UnifiedView hunks={hunks} />}
       <Text>
-        y accept   n reject   q reject & quit   ESC reject
+        y accept   n reject   v toggle side-by-side   q reject &amp; quit   ESC reject
       </Text>
     </Box>
   );
 }
 
+function UnifiedView({ hunks }: { hunks: Hunk[] }): React.ReactElement {
+  return (
+    <Box flexDirection="column">
+      {hunks.map((h, i) => (
+        <HunkLine key={i} hunk={h} />
+      ))}
+    </Box>
+  );
+}
+
 function HunkLine({ hunk }: { hunk: Hunk }): React.ReactElement {
   const sigil = hunk.kind === 'add' ? '+' : hunk.kind === 'del' ? '-' : ' ';
   const color = hunk.kind === 'add' ? 'green' : hunk.kind === 'del' ? 'red' : undefined;
@@ -49,3 +60,68 @@ function HunkLine({ hunk }: { hunk: Hunk }): React.ReactElement {
     </Text>
   );
 }
+
+function pairForSideBySide(hunks: Hunk[]): Array<[Hunk | null, Hunk | null]> {
+  const out: Array<[Hunk | null, Hunk | null]> = [];
+  let i = 0;
+  while (i < hunks.length) {
+    if (hunks[i].kind === 'ctx') {
+      out.push([hunks[i], hunks[i]]);
+      i++;
+      continue;
+    }
+    const dels: Hunk[] = [];
+    const adds: Hunk[] = [];
+    while (i < hunks.length && hunks[i].kind !== 'ctx') {
+      if (hunks[i].kind === 'del') dels.push(hunks[i]);
+      else adds.push(hunks[i]);
+      i++;
+    }
+    const max = Math.max(dels.length, adds.length);
+    for (let j = 0; j < max; j++) {
+      out.push([dels[j] ?? null, adds[j] ?? null]);
+    }
+  }
+  return out;
+}
+
+function SideBySide({ hunks }: { hunks: Hunk[] }): React.ReactElement {
+  const rows = React.useMemo(() => pairForSideBySide(hunks), [hunks]);
+  return (
+    <Box flexDirection="column">
+      {rows.map((row, i) => (
+        <SideBySideRow key={i} left={row[0]} right={row[1]} />
+      ))}
+    </Box>
+  );
+}
+
+function SideBySideRow({ left, right }: { left: Hunk | null; right: Hunk | null }): React.ReactElement {
+  return (
+    <Box flexDirection="row">
+      <Box width="50%">
+        <HalfLine hunk={left} side="left" />
+      </Box>
+      <Text> │ </Text>
+      <Box width="50%">
+        <HalfLine hunk={right} side="right" />
+      </Box>
+    </Box>
+  );
+}
+
+function HalfLine({ hunk, side }: { hunk: Hunk | null; side: 'left' | 'right' }): React.ReactElement {
+  if (!hunk) return <Text> </Text>;
+  const color =
+    hunk.kind === 'add' ? 'green' : hunk.kind === 'del' ? 'red' : undefined;
+  const sigil =
+    hunk.kind === 'add' ? '+' : hunk.kind === 'del' ? '-' : ' ';
+  // For ctx, show ' '; for del on left, show '-'; for add on right, show '+'.
+  // (Cross-cell pollution like 'add' on the left side shouldn't happen given pairForSideBySide.)
+  void side;
+  return (
+    <Text color={color}>
+      {sigil} {String(hunk.lineNo).padStart(4, ' ')}  {hunk.text}
+    </Text>
+  );
+}

tests/tui/Approve.test.tsx

@@ -61,6 +61,33 @@ describe('<Approve>', () => {
     expect(decision).toHaveBeenCalledWith('reject');
   });
 
+  it('toggles to side-by-side on v and shows both halves', async () => {
+    const { stdin, lastFrame } = render(
+      <Approve fileName="x.st" oldText={OLD} newText={NEW} onDecision={() => {}} />
+    );
+    await flush();
+    expect(lastFrame()).not.toContain('│');
+    stdin.write('v');
+    await flush();
+    const out = lastFrame()!;
+    expect(out).toContain('│');
+    expect(out).toContain('counter : INT := 0;');
+    expect(out).toContain('counter : DINT := 0;');
+  });
+
+  it('toggles back to unified on a second v', async () => {
+    const { stdin, lastFrame } = render(
+      <Approve fileName="x.st" oldText={OLD} newText={NEW} onDecision={() => {}} />
+    );
+    await flush();
+    stdin.write('v');
+    await flush();
+    expect(lastFrame()).toContain('│');
+    stdin.write('v');
+    await flush();
+    expect(lastFrame()).not.toContain('│');
+  });
+
   it('calls onDecision("reject") on escape', async () => {
     const decision = vi.fn();
     const { stdin } = render(