feat(proto): add protobuf wire contract and cross-language codegen

proto/noitu/v1/game.proto is the single source of truth for every WebSocket
message. buf generates Go types into server/gen and JavaScript types into
web/src/lib/proto; both trees are committed so building needs no codegen
toolchain.

The Go suite emits binary fixtures into proto/testdata and the JavaScript suite
decodes the same bytes, so the two generated clients are checked against one
artifact rather than against each other's assumptions. CI lints the schema,
rejects breaking changes against main, and fails when the committed generated
trees drift from the schema.

game.NumRejectReasons and game.NumEndReasons let the mapping tests prove every
engine reason has a wire value without guessing where the enum ends.

proto/testdata/server_error.bin

@@ -0,0 +1,2 @@
+J 
+room_not_foundroom_not_found