The checker read any touched contracts module's definitions, skipped the
check that a definition names its own directory's seat, and copied the
indexer's tree reading. sdk/seatdefs now holds the reading and the rule of
which contracts module may define a seat; the indexer's keeping, its
revision seats and the checker's head seats all use it. Two heads defining
one seat fail the judging unless one hands it over to its domain's module.
Claude-Session: https://claude.ai/code/session_01NcbgfFSnn24M3GcqttoU7i
