Refactor prior API to idiomatic S4 accessors

[TimothyWillard]

This change makes prior interaction with SeverityEstimateModel more idiomatic by moving users to explicit S4 getters/replacement setters while preserving a chainable pipeline workflow. It also removes direct public access to the low-level prior helper so external usage is funneled through stable, ergonomic APIs. The result is a cleaner public surface with less duplicated getter logic and aligned docs/tests.

• Add S4 accessor/replacement APIs for all prior slots: active_prior, passive_asymptomatic_prior, passive_symptomatic_prior.
• Add chainable public wrappers: set_active_prior, set_passive_asymptomatic_prior, set_passive_symptomatic_prior.
• Keep setter internals delegated to prior & make prior to enforce official API usage.
• Add internal get_prior helper and route all getter methods through it.
• Standardize unset-getter behavior of warn and return default c(alpha = 1.0, beta = 1.0).
• Update prior documentation/man page to include all new getter/setter/wrapper entries under @rdname prior.
• Update tests to validate public API behavior, including piping, and remove dependency on public prior calls.
• Update NEWS.md example to use set_*_prior in pipelines.