docs: say which fields a box exposes, and narrow the colon rule

[jo16oh]

What changed

Traits

• annotate the box sample with the field it reads, and say the Val's own fields are gone from it:
  p.id is a type error, because dyn drops the concrete type
• a member may not take the name of a field the implementing Val holds, a field another trait on
  that Val requires included. That one is reported at implTrait, not at the declaration
• move A trait is a contract between Vals above the defaults, where the brand is first at stake

The colon rule

The old rule kept a colon wherever the second half gave a reason or a detail, which covers most of
them, and the book ran one mid-sentence colon per eight sentences. The rule now reads what follows
the colon: a noun phrase naming what came before stays, a full clause becomes two sentences, and a
reason becomes because.

• 26 sites across the book follow it. Two become because
• the 9 that stay are all noun phrases, such as the shape: the fields every variant holds