Documentation and release notes for count observation model

[m-murphy]

Parent

#45

What to build

Document the count observation model and count data loading for users adopting the feature in #48 and #47.

Update roxygen for run_mcmc (observation_model argument, when to use "counts", relationship to eps_pos / eps_neg). Update roxygen for load_long_form_data and load_delimited_data (aggregate, reads column). Briefly explain that transmission and latent genotype support-set semantics are unchanged; counts inform the observation layer only.

Add a NEWS.md entry describing the new observation model, loading options, and that defaults remain binary for backward compatibility.

Acceptance criteria

• run_mcmc roxygen documents observation_model and parameter mapping for count mode
• load_long_form_data / load_delimited_data roxygen documents aggregate and reads
• NEWS.md entry added for count observation model feature
• Documentation accurately reflects shipped behavior from Observation model interface with binary parity #46–Count Poisson observation model — full MCMC inference path #48 and Count data loading and genotyping initialization #47 (no aspirational APIs)

Blocked by

• Count Poisson observation model — full MCMC inference path #48