Mark complete() obsolete and move page code to onChange

[jwrosewell]

What to change

Mark complete() obsolete in the client script and move page code, examples
and documentation to onChange(). Keep complete() working, because pages
in the wild use it, and remove it only in a major release.

Why

complete(fn) answers a question that stopped having one answer when the
script gained refresh().

Read at commit f47917c of this repository, on branch
feature/create-last-a (#23):

• complete(fn) (line 1364) calls fn at once when completed or failed
  is already true. Otherwise it registers a wrapper through onChange
  (line 1389) that calls fn when the round ends.
• refresh() (line 1400) sets completed and failed back to false and
  runs another round, which happens when the visitor's answer arrives or
  changes.
• loadParsedJSON (line 982) sets completed true and fires every
  changeFuncs handler at the end of every round.

So a handler registered before the first round ends stays in changeFuncs
and fires again at the end of every later round, whilst one registered after
that fires once, immediately, and never again. Which of the two a page gets
depends on when its code ran, which a page author cannot control on a slow
network.

The result that matters: on the ordinary path the visitor answers after the
first round has ended, so the 51Did exists only in a later round. A handler
registered late, which is what a documentation example does, is called with
a payload that carries no 51Did.

onChange(fn) has none of this. It fires on every payload, including one
that arrives after a refresh, and it catches a handler up when the page view
was served from the session storage cache (line 1351).

What this needs

1. A note in this repository's README and in the rendered script's own
   documentation saying complete() is obsolete and what to use instead.
2. The same in each pipeline language's documentation, because the script is
   rendered by every one of them.
3. Examples and demonstrations move to onChange, checking the value is
   present before using it. The 51Degrees developer documentation is being
   changed to do that now
   (documentation#238).
4. A decision on whether complete() should fire once per page view until
   it is removed, which is what it did before refresh() existed.

Raised on James Rosewell's decision of 15 September 2026, during the create
last review, which is tracked internally.