add diff_atom_cmp() and error handling to diff_atom_same()

avoid include statements in header files; make each .c pull in what's needed

replace enum diff_rc errors with plain errno values

fall back on file i/o in case an input file cannot be memory-mapped

move 'struct range' and related functions into the diff_ namespace no functional change

KNF: use max 80 columns

KNF: function declaration return types on separate line

Fix macro diff_atom_*idx return value types The macros sometimes returned unsigned int (atoms.len) or long int ((ATOM) - head). Make that always unsigned int, with some range checks against negative values.

patience: reduce sections by swallowing identical lines before LCS

move struct range defs to diff_main.h (prep for upcoming patch)

initial commit