A module had one channel back to its caller: the int returned from
dpm_module_execute, handed through by dpm_execute. Any detail behind
that number could only reach a log, so a caller wanting the reason had
to read output rather than ask for it.
dpm_set_last_error joins the exported API. A module records its reason
on the context handed to its entry point, which belongs to the caller,
and the caller reads it back with dpm_get_last_error.
The two accessors on the context now say what they do to it:
dpm_module_path becomes dpm_get_resolved_module_path, naming the value
it reports rather than the setting it came from, and dpm_last_error
becomes dpm_get_last_error, pairing with the setter.
Path normalization moves to sanitizers.cpp, which holds the conversions
that put a value written by a person into the single form the library
stores it in.
The Doxygen configuration, the page list, and the docs target live in
docs/, and the written documents live in docs/PROSE/, leaving the
top-level file with the output paths and the add_subdirectory call.
The subdirectory is given its own binary directory so that CMake's
scaffolding stays out of the documentation output, which holds pdf/ and
html/ and nothing else.
Paths both files need are set once above the call: the child's output
and cleanup and the parent's clean rule refer to the same variables.
