Bundled mksquashfs only supports zstd compression despite --comp parameter accepting other formats

## Issue: --comp parameter doesn't work with gzip/xz/lzo/lz4

Why other compression methods are not added?

### Problem
Using `--comp gzip` fails because the bundled mksquashfs only supports zstd:

```
mksquashfs: Compressor "gzip" is not supported!
mksquashfs: Compressors available:
zstd (default)
```

### Cause
In `ci/install-static-mksquashfs.sh`, mksquashfs is built with only zstd enabled:
```bash
GZIP_SUPPORT=0 XZ_SUPPORT=0 LZO_SUPPORT=0 LZ4_SUPPORT=0 ZSTD_SUPPORT=1
```

### Suggested Fix
Either:
- Enable all compression formats in the build script, OR
- Validate `--comp` parameter and reject unsupported formats with clear error message