These were all scaffolding that drifted out of use, and each one costs
a reader time before they discover it does nothing:

- UUIDASStringSerializer duplicated UUIDSerializer byte for byte; the
  differing descriptor name never reaches the JSON/YAML wire format, so
  the choice between them was a coin flip for contributors.
- Velocity's BuildInfo was never referenced (the plugin reads its version
  from PluginContainer) and read a "commit" property the build never
  wrote, so it would have reported "unknown" had anyone called it.
- KanaConverter.TrieNode.Leaf is never constructed: buildTrie starts from
  a Branch and insert only ever returns Branch. Six branches guarded
  against a state the type system allowed but the code could not produce.
  With those gone, isValidRomaji and toHiragana were visibly the same
  trie walk, so they now share one longestMatch.
- @Deprecated command handling had no annotated command to act on.
- The settings backup restore looked for *.backup.* files that nothing in
  the repository writes, so it always fell through to empty settings.

Also drops CommandContext.replyWithEvent/replyPlain, PluginCoroutineScope's
unused plugin parameter, GitHubRelease fields no caller reads, and four
language keys with no lookup site.

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

Allow /tell and /reply to reach players on other Paper servers behind a
Velocity proxy using the "<player>@<server>" target syntax.

Engine (protocol bumped 1.0.0 -> 1.0.1, optional sub-channels):
- Add DirectMessageRelay, DirectMessageError, PresenceSnapshot/PresenceEntry
  and PresenceRequest messages plus codec branches.

Velocity:
- CrossServerDirectMessageRelay routes a DM to the target server (or returns
  a delivery error to the source).
- PresenceTracker broadcasts proxy-wide presence snapshots on join/quit/switch
  and on request.

Paper:
- RemotePlayerRegistry caches proxy presence for completion and remote target
  resolution.
- CrossServerDirectMessageManager handles send/receive/error and dedup.
- DirectMessageHandler reply state generalized to ReplyTarget (Local/Remote)
  so /reply works across servers.
- TellCommand parses "name@server", completes local names and remote
  name@server targets, and uses exact local name matching.
- New crossServerDirectMessage config flag and i18n keys (en/ja).

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