feat: show which album is copying, and how far through

The transfer looked hung. rsync prints nothing while it builds its file list,
which on fifty thousand files over USB is several minutes of silence, and
--info=progress2 does not help: with incremental recursion its percentage is
computed against a list rsync has not finished discovering, so it moves
backwards as often as forwards.

The script now counts what needs copying first and says so, then renders its
own single line that rewrites in place, showing the album currently going
across and a percentage against a total that is actually known. Counting costs
a second pass over the tree. That is the price of a percentage meaning
something, and it is cheaper than staring at a blank terminal wondering whether
the thing has died.

Directories are excluded from the count. rsync reports those too, and including
them puts the figure past a hundred per cent.

Piped to a log the line becomes a plain one every thirty seconds, because a log
full of carriage returns and escape codes is not a log anybody reads.

README.md

@@ -163,6 +163,19 @@ and the various filesystem metadata directories from deletion — the mirror doe
 not contain them, and without the exclusion a sync to the card root would
 remove the Rockbox install.
 
+Progress is a single line that rewrites itself:
+
+```
+[ 12,345 / 49,600  24%] King Gizzard & the Lizard Wizard / PetroDragonic Apoc…
+```
+
+rsync says nothing at all while it builds its file list, which on fifty
+thousand files over USB is minutes of apparent hang, and its own `progress2`
+percentage is computed against a list it has not finished discovering. So the
+script counts first — a second pass over the tree, which is what a percentage
+that means something costs — and renders the rest itself. Piped to a log it
+prints a plain line every thirty seconds instead, with no carriage returns.
+
 The unmount is the point of doing this in a script. FAT32 has no journal and
 the device is reached through disk mode, so an interrupted write is corruption
 that needs `fsck.vfat` from another machine.