wasm/js/emscripten: Fix page loading race

wasm/js/emscripten: Fix page loading race

Using a stunt webserver which artificially introduces a 3s delay just
before the last line of the HTML output, I have reproduced a
uwer-reported loading/startup race bug:

Previously the wasm loading was started by the <script> element,
synchronously. If the wasm loading is fast, and finishes before the
HTML loading, the onRuntimeInitialized event may occur before
initPuzzles.  But initPuzzles sets up the event handler.

Fix this bug, and introduce a new comment containing an argument for
the correctness of the new approach.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>