docs.to-html throwing permission denied (Read-only file system) exception

[aryairani]

Describe and demonstrate the bug

% rm -rf /tmp/base
% ucm
@unison/base/main> docs.to-html : /tmp/base
Encountered exception:
/doc.html: withFile: permission denied (Read-only file system)
@unison/base/main> 

Not out of space:

% df -h /tmp
Filesystem      Size    Used   Avail
/dev/disk3s1   460Gi   405Gi    18Gi

Not so read-only as to prevent writing the first 217 docs:

% du -chs /tmp/base
700K    /tmp/base
700K    total
% find /tmp/base | wc -l
     217

blocking re-testing #2608

Environment (please complete the following information):
v0.5.23, macOS 14.5 (23F79)