fix(changes): a huge line no longer cuts the diff short; preview pages up to 64 MB

A page that inlines a video as base64 has rows of megabytes. Its patch was read
at the 200 KB per-file cap, so the first such row used the whole budget and
every hunk after it was lost ("Diff cut short"). The patch is now read up to
16 M characters, rows over 4,000 characters are cut to 1,000 with an `elided`
count the diff shows, and only then is the cap applied. Hunk actions re-read
git's own diff by header and are unchanged.

The HTML preview refused a page over 10 MB and showed only the file icon; a
self-contained 18 MB slide deck now opens (limit 64 MB).

server/__tests__/git-diff-ops-hardening.test.js

@@ -473,6 +473,55 @@ describe("reads larger than the git() buffer report truncated, not missing", ()
   });
 });
 
+describe("a page with megabyte-long lines (inlined base64) keeps every hunk", () => {
+  const blob = (c) => c.repeat(3 * 1024 * 1024); // one 3 MB line
+
+  it("cuts the long rows down and still shows the hunks after them", async () => {
+    const body = (mid, end) =>
+      [
+        "<h1>title</h1>",
+        ...Array.from({ length: 40 }, (_, i) => `row ${i}`),
+        mid,
+        ...Array.from({ length: 40 }, (_, i) => `tail ${i}`),
+        end,
+        "",
+      ].join("\n");
+    write(LANE, "deck.html", body(`<video src="${blob("A")}">`, "<p>old end</p>"));
+    g(LANE, "add", "-A");
+    g(LANE, "commit", "-qm", "deck");
+    write(
+      LANE,
+      "deck.html",
+      body(`<video src="${blob("B")}">`, "<p>new end</p>").replace(
+        "<h1>title</h1>",
+        "<h1>new title</h1>"
+      )
+    );
+    const out = await collectChanges(LANE, { range: "work", file: "deck.html" });
+    const f = out.files[0];
+    assert.equal(f.truncated, undefined);
+    assert.equal(out.truncated, false);
+    const lines = f.hunks.flatMap((h) => h.lines);
+    const long = lines.filter((l) => l.elided);
+    assert.equal(long.length, 2);
+    for (const l of long) {
+      assert.ok(l.text.length < 2000);
+      assert.equal(l.text.length + l.elided, '<video src="'.length + 3 * 1024 * 1024 + 2);
+    }
+    // The change AFTER the 3 MB rows survives: the cap no longer eats it.
+    assert.ok(lines.some((l) => l.type === "add" && l.text === "<p>new end</p>"));
+    assert.ok(lines.some((l) => l.type === "add" && l.text === "<h1>new title</h1>"));
+  });
+
+  it("leaves ordinary rows whole", async () => {
+    write(LANE, "a.txt", "one\nTWO\n");
+    const out = await collectChanges(LANE, { range: "work", file: "a.txt" });
+    const lines = out.files[0].hunks.flatMap((h) => h.lines);
+    assert.ok(lines.every((l) => l.elided === undefined));
+    assert.ok(lines.some((l) => l.text === "TWO"));
+  });
+});
+
 describe("loop 2: secret file at any depth, symlinked .claude", () => {
   const KEY = '{"env":{"ANTHROPIC_API_KEY":"sk-deep"}}';