HHH4 joint modelling

[CyGei]

HHH4 endemic-epidemic model: allowing users to fit and forecast models that consider all series together, rather than independently. We may reuse this architecture for other joint modelling methods (@shandersk, see if this could help with LEMMA?)

• Introduced infrastructure for joint models via new functions in R/joint.R, including new_joint_model() for specification, forecast_joint() for fitting and forecasting, and helper joint_to_long() for formatting results.
• Added S3 methods and export for the new HHH4 model, enabling it to work seamlessly with the package’s forecasting and reporting functions.
• Added a comprehensive vignette (vignettes/hhh4.Rmd) showing how to use HHH4.
• Updated package dependencies to include rlang and suggest surveillance (required for HHH4).