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.test.js

@@ -203,18 +203,21 @@ describe("collectChanges — work range", () => {
     });
   });
 
-  it("still returns content when a huge file has no newline to cut on", async () => {
-    // A minified bundle is ONE line. Cutting the patch at the last newline
-    // inside the cap would land at index -1 here and hand back an empty
-    // patch — a file changed by a megabyte rendering as "no textual change".
+  it("still returns content when a huge file is one line", async () => {
+    // A minified bundle is ONE line. It used to be cut at the per-file cap
+    // (and, cut at the last newline, could come back empty — a file changed
+    // by a megabyte rendering as "no textual change"); now the row itself is
+    // cut down and says how much it hides, and nothing is truncated.
     const { MAX_FILE_BYTES } = require("../lib/git-diff");
     write(LANE, "min.js", `${"x".repeat(MAX_FILE_BYTES + 5000)}\n`);
     const out = await collectChanges(LANE, { range: "work", file: "min.js" });
     const entry = out.files[0];
-    assert.equal(entry.truncated, true);
-    assert.equal(out.truncated, true);
-    assert.ok(entry.hunks.length > 0, "a truncated patch still carries its hunk");
-    assert.ok(entry.hunks[0].lines[0].text.startsWith("xxx"));
+    assert.equal(entry.truncated, undefined);
+    assert.equal(out.truncated, false);
+    assert.ok(entry.hunks.length > 0, "the patch still carries its hunk");
+    const row = entry.hunks[0].lines.find((l) => l.type === "add");
+    assert.ok(row.text.startsWith("xxx"));
+    assert.equal(row.text.length + row.elided, MAX_FILE_BYTES + 5000);
   });
 
   it("reports a binary file without trying to diff it", async () => {