fix(odoc): fix OxCaml test infrastructure issues
- Fix duplicate install rule in doc/dune (logo installed by both
documentation and install stanzas)
- Add OxCaml cppo rule to test/xref2/lib/dune passing -D OXCAML flag
- Update common.cppo.ml for OxCaml Typemod API changes
(type_interface, type_implementation signatures) and add
source_loc_jane field
- Add Strengthen pattern to odoc_print.ml signature_of_module_type_expr
- Fix cram test expected indentation in oxcaml_modes.t
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>