Channels: fix post propagation, discussions enforcement, sync reliabi…

…lity

Post propagation
- Subscriber now skips already-known posts before chain validation, so an
  open channel that re-sends its full history each pull no longer rejects the
  batch on the first known post — every post reaches subscribers, not just the
  first
- Apply the verified manifest and the valid post prefix even when a later post
  fails validation, so one bad post can no longer wedge a subscriber

Discussions / acks
- Honour the publisher's comment and reaction acknowledgement: a rejected
  reply or reaction surfaces a failure instead of appearing as sent
- Only raise a comment notification for genuinely new comments, and only
  refresh on genuinely changed reactions — stops repeated notifications and
  feed churn from full re-sends

Reliability
- Re-bind the channel onion when the transport recovers, reusing the persisted
  key so the address is unchanged
- Skip scheduling a pull for a channel whose previous pull is still in flight

Discussions propagation (gated, default off)
- Optional publisher-signed discussions flag in the channel manifest, behind
  ChannelConstants.DISCUSSIONS_IN_MANIFEST; wire-inert until enabled in
  coordination with the other client

v2.0.1: vendor PhotoView 2.3.0 source

Replaces the vendored zerion-android/libs/PhotoView-2.3.0.aar (which was
never tracked in git) with PhotoView's Java source under
zerion-android/src/main/java/com/github/chrisbanes/photoview/. Same code,
same package path, identical call sites.

Required for F-Droid main repository inclusion with reproducible builds.
Both the official F-Droid VPS build and F-Droid's CI now run the same
compilation path, so the resulting APK matches byte-for-byte and the
signing key stays the same across Play Store, fdroid.zerion.chat, and
F-Droid main. Users can switch between channels without reinstalling.

PhotoView is licensed Apache 2.0 (LICENSE preserved alongside source).
No functional change.

versionCode 20000 -> 20001
versionName 2.0.0 -> 2.0.1

Zerion v2.0.0

Zerion v1.7.0

Merge dev into master for v1.7.0 release

Zerion 1.6.2

Zerion v1.6.1 — security audit point release

Drop history-tombstone comments left in MessageTypes after 9735c14

Reservation comments explaining why values 5 and 6 are missing don't
belong in source. Removed entries that reference commit hashes and prior
state. Reservation intent stays in the commit history of 9735c14 and in
the iOS handoff thread, not as inline noise.

Zerion 1.5.0 — F-Droid signed release (B.3 + B.4 + Vault + audit fixes)

Zerion v1.5.0 — B.3 in-band hybrid-key signing enabled

Joint Android+iOS release. CONTACT_INFO grows to 5 slots with an
Ed25519 proof binding the long-term ML-KEM-768 portion of each peer's
static hybrid identity, closing the trust-on-first-use post-quantum
downgrade path. messaging.minorVersion = 5.

ML Kit barcode scanner replaced by ZXing core, eliminating the
Firebase phone-home over clearnet HTTPS.