refactor: extract pkg/charset and internal/event packages

Extract ConvertChunk/FormatHexFrame into pkg/charset (zero external deps).
Extract UIEvent/UIEventKind/UIPanelKind types into internal/event.
Update all references across main package to use qualified imports.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

plugins/demo.lua

@@ -0,0 +1,14 @@
+-- Demo Lua plugin for the runtime plugin system.
+-- It is shipped disabled by default and only runs after `.plugin load`.
+
+function OnInput(payload)
+  return payload
+end
+
+function OnOutput(payload)
+  return payload
+end
+
+function OnCommand(line)
+  return line, true
+end