keepGainMap(): extract() crops the base image but leaves the gain map uncropped

Nobody has claimed this yet.

### Is this a possible bug in a feature of sharp, unrelated to installation?
- [ x] Running `npm install sharp` completes without error.
- [ x] Running `node -e "import 'sharp'"` completes without error.

### Are you using the latest version of sharp?
- [ x] I am using the latest version of `sharp` as reported by `npm view sharp dist-tags.latest`.
Running 0.35.4

### What is the output of running `npx envinfo --binaries --system --npmPackages=sharp --npmGlobalPackages=sharp`?
System:
OS: macOS 27.0
CPU: (18) arm64 Apple M5 Max
Memory: 6.47 GB / 128.00 GB
Shell: 3.2.57 - /bin/bash
Binaries:
Node: 24.18.0 - /Users/redacted/.nvm/versions/node/v24.18.0/bin/node
Yarn: 1.22.19 - /usr/local/bin/yarn
npm: 11.16.0 - /Users/redacted/.nvm/versions/node/v24.18.0/bin/npm

### Does this problem relate to file caching?
- [x ] Adding `sharp.cache(false)` does not fix this problem.

### Does this problem relate to images appearing to have been rotated by 90 degrees?
- [x ] Using `rotate()` or `keepExif()` does not fix this problem.

### What are the steps to reproduce?
run code below

Tested against sharp main at commit 9dcb3dd (reports version 0.35.4), using libvips 8.18.6 and libultrahdr 2.0.2.

### What is the expected behaviour?
The image should be cropped, but show no changes in the cropped area. However, when you view the output on an HDR-capable display, you'll see ghosting because the gain map was not cropped to stay aligned to the base image.

Expected result: both the base image and this source’s full-resolution gain map are cropped to 601 × 499.
Actual: the base becomes 601 × 499, but the gain map remains 1080 × 1080.

### Please provide a minimal, standalone code sample, without other dependencies, that demonstrates this problem
```
node - "./ISO JPG - sRGB - transcoding test - Greg Benz.jpg" <<'NODE'
const sharp = require('sharp');
const path = require('node:path');

(async () => {
const output = path.resolve('./crop-gainmap-issue.jpg');

await sharp(process.argv[2])
.keepGainMap()
.extract({ left: 101, top: 153, width: 601, height: 499 }) // Both images should be cropped, but the gain map stays at its original size.
.toFile(output);

const base = await sharp(output).metadata();
const map = await sharp(base.gainMap.image).metadata();

console.log(`Base image: ${base.width} × ${base.height}`);
console.log(`Gain map: ${map.width} × ${map.height}`);
console.log(`Saved: ${output}`);
})().catch(error => {
console.error(error.message);
process.exitCode = 1;
});
NODE
```

### Please provide sample image(s) that help explain this problem
Source to test:

The failed output:

Run the provided Node.js reproduction with keepGainMap(), extract(), and toFile(), then inspect the resulting metadata through base.gainMap.image. Trace the keepGainMap/extract interaction in the sharp entry points and confirm the behavior against the supplied 601 × 499 base and 1080 × 1080 gain-map dimensions. Done means the gain map remains aligned with the cropped base image.

Written by the indexing model from the issue text.