refactor: single-source the plugin messaging channel and dedup cache

The channel Paper and Velocity talk over was declared in seven places, in
two spellings ("lunaticchat:main" and the namespace/name pair), one of
them an inline literal in CrossServerChatManager that bypassed even its
own file's constant. Renaming it meant finding all seven; missing one
leaves both sides compiling and starting, just not talking. It now lives
next to the codec that defines the wire format.

The echo-suppression cache was likewise written twice, and the copies had
already drifted in style - one hand-rolled the expiry sweep, the other used
filter/map - while staying semantically identical. Any future change to
eviction would have had to land in both, and CrossServerChatManager's copy
carried a comment claiming ConcurrentHashMap iterators cannot remove(),
which they can.

MessageDeduplicationCache documents the one property that surprised the
tests written against it: eviction orders by millisecond timestamp, so a
burst inside a single millisecond evicts arbitrarily among its members.

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

0 files changed, 0 insertions, 0 deletions