Proposed resolution:

Add the above declaration as the first line after the comment in [rand.adapt.ibits] p4:

// engine characteristics
static const size_t word_size = w;

and accept my apologies for the oversight.

In preparing N2111, an error on my part resulted in the omission of the following line from the template synopsis in the cited section:

static const size_t word_size = w;

(This same constant is found, for example, in 26.4.3.3 [rand.eng.sub].)