v0.16.0

One new component. Additive; no existing default changes.

box-formatted-duration

How long something takes or lasts, in the reader's locale — the fifth member of
the formatted-value family, and the one most obviously missing in a content
platform. Retention periods, SLAs and processing times are all durations, and
every host was rendering them by hand.

<box-formatted-duration value="5400"></box-formatted-duration>   <!-- 1 hr, 30 min -->

value takes a count of seconds or an ISO 8601 duration, because hosts have
both: an API field is usually a number, while <time datetime> wants the ISO
form. Whichever comes in, the ISO form goes back out, so the exact quantity
survives for anything reading the document rather than looking at it.

format-style takes short (default), long or narrow, and max-units
controls how much precision to show — two by default, since a duration carries
more than a reader wants to scan.

What it refuses is as deliberate as what it formats:

• A negative value hides the element. A duration carries no direction, so a
  negative one is a host bug; box-relative-time is what expresses "ago".
• Months and years are refused. A month is not a fixed number of seconds,
  and P1M is ambiguous with a minute besides, so rendering either as a fixed
  span would misstate how long it actually is.

Intl.DurationFormat renders it where the browser has it. That API is recent,
so where it is absent the same output is composed from Intl.NumberFormat's
unit style and Intl.ListFormat; the two paths were compared across styles and
locales before the fallback was relied on, and a test runs it with
Intl.DurationFormat deleted.

Two behaviours came out of testing rather than design, and both are the kind
that would have shipped silently:

• A trailing zero unit is dropped, so an exact hour is "1 hr" rather than
  "1 hr, 0 min". Only trailing ones — an interior zero is load-bearing, since
  dropping it would let "1 day, 0 hr" become "1 day, 5 min" and claim a
  precision the two-unit split does not have.
• A zero duration renders "0 sec", because Intl formats one to the empty
  string, which would leave a visible element with nothing in it. The option
  that forces that appends "0 sec" to every duration in browsers honouring it,
  which Node's Intl quietly does not — so it passed in jsdom while being
  visibly wrong in a browser. It is now scoped to the zero case.