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>

1 files changed, 16 insertions, 0 deletions

diff --git a/engine/src/main/kotlin/dev/m1sk9/lunaticChat/engine/protocol/PluginMessageChannel.kt b/engine/src/main/kotlin/dev/m1sk9/lunaticChat/engine/protocol/PluginMessageChannel.kt
new file mode 100644
index 0000000..85093ed
--- /dev/null
+++ b/engine/src/main/kotlin/dev/m1sk9/lunaticChat/engine/protocol/PluginMessageChannel.kt
@@ -0,0 +1,16 @@
+package dev.m1sk9.lunaticChat.engine.protocol
+
+/**
+ * The plugin messaging channel Paper and Velocity exchange [PluginMessage]s over.
+ *
+ * Both sides must agree on this exactly. Declaring it next to the codec keeps a rename from
+ * silently splitting the two halves of the protocol: a Paper server and a proxy that disagree
+ * still compile and start, they just stop talking.
+ */
+object PluginMessageChannel {
+    const val NAMESPACE = "lunaticchat"
+    const val NAME = "main"
+
+    /** The channel in Bukkit's `namespace:name` form. */
+    const val ID = "$NAMESPACE:$NAME"
+}