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).

docs/API.md

@@ -817,6 +817,11 @@ that file.
 - Three caps guard the response: **300** files listed, **200 KB** of patch per
   file, **1 MB** of patch in a `full` read. A cap that bit sets `truncated` on
   the response and on the file it cut.
+- A row longer than **4,000** characters (an inlined base64 image or video, a
+  minified bundle) comes back cut to its first 1,000 with `elided: <characters
+  hidden>`, BEFORE the per-file cap is applied, so one huge line no longer eats
+  the hunks after it. Each file's raw patch is read up to 16 M characters; past
+  that it is still `truncated`.
 - Read-only, no same-origin guard. `available: false` (HTTP **200**) for a cwd
   git cannot read; `range=branch` compares against the workspace's base branch,
   else the repo's default branch (what `origin/HEAD` names, else `main`/`master`)