Bugfix: figure URLs that use GitHub's leading-slash convention
(![](/images/foo.png)) now resolve correctly when the markdown lives in
a subdirectory like sources/. Both the default mode (relpath rewriting)
and --copy-figures (bundle into build/figures/) were broken for this
layout in v0.8.0 — resolution tried only source_dir/url and silently
gave up when the file wasn't there. v0.8.1 adds cwd as a fallback
resolution root (cwd = texmark's invocation directory), matching the
GitHub-style "/foo means project root" intent.