tui: add line-based diff hunk computation

computeHunks() returns Hunk[] (add | del | ctx) with line numbers on
the new side for add/ctx and the old side for del.

Both inputs are normalized to end with a newline before calling
jsdiff's diffLines, otherwise a missing EOF newline is treated as a
token boundary and the last line shows up as a spurious del+add
pair.

src/tui/shared/diff.ts

@@ -0,0 +1,39 @@
+import { diffLines } from 'diff';
+import { Hunk } from './types.js';
+
+export function computeHunks(oldText: string, newText: string): Hunk[] {
+  // jsdiff treats trailing-newline presence as a token boundary, so a missing
+  // EOF newline shows up as a spurious del+add pair on the last line. Normalize.
+  const a = oldText.endsWith('\n') ? oldText : oldText + '\n';
+  const b = newText.endsWith('\n') ? newText : newText + '\n';
+  const parts = diffLines(a, b);
+  const out: Hunk[] = [];
+  let oldLine = 1;
+  let newLine = 1;
+
+  for (const part of parts) {
+    const lines = part.value.split('\n');
+    if (lines.length > 0 && lines[lines.length - 1] === '') {
+      lines.pop();
+    }
+    if (part.added) {
+      for (const text of lines) {
+        out.push({ kind: 'add', lineNo: newLine, text });
+        newLine++;
+      }
+    } else if (part.removed) {
+      for (const text of lines) {
+        out.push({ kind: 'del', lineNo: oldLine, text });
+        oldLine++;
+      }
+    } else {
+      for (const text of lines) {
+        out.push({ kind: 'ctx', lineNo: newLine, text });
+        oldLine++;
+        newLine++;
+      }
+    }
+  }
+
+  return out;
+}

tests/tui/diff.test.ts

@@ -0,0 +1,47 @@
+import { describe, it, expect } from 'vitest';
+import { computeHunks } from '../../src/tui/shared/diff.ts';
+
+describe('computeHunks', () => {
+  it('returns ctx hunks when both sides are identical', () => {
+    const a = 'one\ntwo\nthree';
+    const hunks = computeHunks(a, a);
+    expect(hunks.every((h) => h.kind === 'ctx')).toBe(true);
+    expect(hunks).toHaveLength(3);
+  });
+
+  it('marks added lines', () => {
+    const a = 'one\ntwo';
+    const b = 'one\ntwo\nthree';
+    const hunks = computeHunks(a, b);
+    const added = hunks.filter((h) => h.kind === 'add');
+    expect(added).toHaveLength(1);
+    expect(added[0].text).toBe('three');
+  });
+
+  it('marks deleted lines', () => {
+    const a = 'one\ntwo\nthree';
+    const b = 'one\ntwo';
+    const hunks = computeHunks(a, b);
+    const deleted = hunks.filter((h) => h.kind === 'del');
+    expect(deleted).toHaveLength(1);
+    expect(deleted[0].text).toBe('three');
+  });
+
+  it('handles a substitution as del + add', () => {
+    const a = 'a\nb\nc';
+    const b = 'a\nB\nc';
+    const hunks = computeHunks(a, b);
+    expect(hunks.some((h) => h.kind === 'del' && h.text === 'b')).toBe(true);
+    expect(hunks.some((h) => h.kind === 'add' && h.text === 'B')).toBe(true);
+  });
+
+  it('reports add/ctx line numbers against the new side, del against the old side', () => {
+    const a = 'a\nb\nc';
+    const b = 'a\nx\nc';
+    const hunks = computeHunks(a, b);
+    const del = hunks.find((h) => h.kind === 'del')!;
+    const add = hunks.find((h) => h.kind === 'add')!;
+    expect(del.lineNo).toBe(2);
+    expect(add.lineNo).toBe(2);
+  });
+});