New — Save a clip straight from the wall

A ⭳ button now sits on every video tile in the wall, beside the ❚❚.
It saves that one clip the way the ⭳ under a post does: the video and
its separate reddit sound track muxed into a single mp4, no external
tools. A clip already playing on the wall is saved from the copy
already in hand, so it usually costs nothing but the mux. Press it
again to cancel.

Picture tiles don't get one — the ⭳ under the post already answers
that. On very small tiles the button steps aside rather than landing
on the speaker; open the tile full screen and it's there.

Fixed — clips that claimed to have no sound

Players built before their audio track arrived asked the video element
whether it had sound, and on v.redd.it the answer is always no by
design — the sound is a separate file. So players muted themselves and
greyed out working controls while a loaded track played beside them.
On a cold /r/all this affected most players on the page. They now
notice when the track turns up and change their minds.

Fixed — waveforms that never appeared

A waveform that failed to load once was remembered as "this clip is
silent" and never attempted again, which disabled the ∿ button for the
rest of the session. Usually nothing was wrong with the clip — it had
simply asked at the moment sixteen other clips were downloading. Now a
failure is treated as a failure: it retries a few times, further apart
each time, and every undrawn waveform gets another chance the moment
the download queue goes quiet.

Two related fixes: a clip loaded from cache no longer removes itself
from the redraw list, and a decode that never returns now gives up
instead of holding its slot for the life of the page. The time limit
is much longer on Android, where a slow decode is normal rather than a
fault.

Fixed — restored page stacks forgot they were a run

Pages brought back from a saved session looked correctly numbered but
were actually unrelated rows that each believed it was page 1 of its
own stack. Stepping back to a page already on screen re-fetched it
instead of reusing it, and paging onward renumbered the run — five
pages could become two. Pages now remember how they connect, and the
run is rebuilt properly when the session is restored.

Also fixed: unlocking a page in the middle of a run no longer
renumbers the whole run.