The handler table was a package-level global populated by init(), making
registration implicit and untestable. Move it to buildHandlerTable()
which returns the map, store it as a Server struct field initialized in
NewServer(), and add a missing-handler guard in handlePacketGroup to log
a warning instead of panicking on unknown opcodes.
Port test files from v9.2.x-stable branch to increase channelserver
coverage from 13.8% to 25.6% (556 tests passing).
Adapted all files to main's struct definitions: config import alias,
Airou/CatDefinition rename, packet field mismatches, Raviente struct
differences, and maxPlayers defaults. Removed tests referencing
production code not yet on main (Player, FestivalColour, etc.).
Excluded handlers_register_test.go (Raviente completely redesigned).
