Updater

feat(updater): tag-tracked updater deployment + bump to updater-v2

EnsureUpdaterAsync now writes the deployed updater tag to
%APPDATA%/A2Meter/updater_tag.txt and re-downloads whenever the
constant `UpdaterReleaseTag` doesn't match what's on disk. This
unblocks the self-rechecking updater rewrite: existing clients with
the old A2Updater.exe automatically get replaced on next launch
when we bump the tag.

Also download to a .new temp file and atomically replace, so a
dropped connection during download can never leave a half-written
A2Updater.exe behind.

Bump tag to `updater-v2`. Action item: publish a GitHub release
with that tag whose A2Updater.exe asset is the new self-contained
single binary.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>