Close high-severity findings from the security review: default-deny blob
gets for unmapped hashes, map collection members for ACL, fail closed on
unknown access modes, require vnd1 tickets only, redact tickets from events,
exclude Android app-data backups, and tighten secret-file creation.
Default Android receive destination is now the shared system Downloads
collection (like desktop), using MediaStore on API 29+ instead of
app-private storage. Legacy app-private defaults are promoted back to
public Downloads so existing installs pick up the fix.
Tighten cold-open and in-app invitation handling so ticket acquisition is
stricter and less racey across hosts, and expand automated coverage for the
receive history and acquisition flow before merge.
