Per-distribution S3 interface: migrate gamma (PoC for #41)

[sbfnk-bot]

Proof-of-concept for the per-distribution refactor (#41, first checklist box). It introduces the dispatch machinery and migrates gamma only, so we can see the pattern (and review) on something small before doing the rest.

What it does

• R/distribution.R — new_dist(name, params) dispatch objects + the interface generics (dist_natural_params, dist_cdf, plus reuse of the existing mean/sd generics). Discretisation is an optional capability (dist_cdf.default errors), not a type split.
• R/gamma.R — everything gamma in one file: dist_natural_params.gamma, dist_cdf.gamma, mean.gamma, sd.gamma.
• The gamma arms of the discrete_pmf (pdist), natural_params, mean.dist_spec and sd.dist_spec switches now delegate to those methods. Every other distribution is untouched.

Notes / scope

• Behaviour-preserving: full test suite green, lint_package() = 0. mean/sd/natural_params/discretise for gamma verified identical.
• Kept to internal generics (no public-API change) for this slice. mean/sd are used directly (they're generics); natural_params's own dispatch and the stored-component representation change are deliberately deferred.
• Once every distribution is migrated, each switch collapses to a single <generic>(new_dist(distribution)) call and disappears — and R/template.R + the "how to add a distribution" doc follow.

Draft, for design feedback on the shape before the remaining distributions.

Summary by CodeRabbit

• New Features

  • Added support for gamma distribution calculations, including mean, standard deviation, and CDF-based operations.
  • Expanded distribution handling so more calculations now use a consistent dispatch mechanism.

• Bug Fixes

  • Improved gamma distribution support in discretised calculations, making results more consistent across related functions.

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Walkthrough

This PR introduces a new distribution dispatch interface (new_dist, dist_natural_params, dist_cdf) in R/distribution.R, implements it for the gamma distribution in R/gamma.R, and refactors gamma-specific logic in R/dist_spec.R (mean, sd, natural_params, discrete_pmf CDF) to delegate through this interface instead of inline formulas. NAMESPACE is updated accordingly.

Changes

Gamma distribution dispatch interface

Layer / File(s)	Summary
Distribution dispatch interface R/distribution.R	Adds new_dist() constructor, dist_natural_params() and dist_cdf() generics, and dist_cdf.default() which errors via cli::cli_abort() when a distribution has no CDF method.
Gamma distribution implementation R/gamma.R	Implements dist_natural_params.gamma, dist_cdf.gamma (using pgamma), mean.gamma (shape/rate), and sd.gamma (sqrt(shape/rate^2)).
Gamma delegation in dist_spec.R R/dist_spec.R	Replaces inline gamma formulas in discrete_pmf(), mean.dist_spec(), sd.dist_spec(), and natural_params() with calls through new_dist("gamma", ...) and the dist_cdf/dist_natural_params interface.
NAMESPACE registrations NAMESPACE	Registers new S3 methods for dist_cdf (default, gamma), dist_natural_params (gamma), mean (gamma), and sd (gamma).

Possibly related issues

• Refactor dist_spec.R into a per-distribution S3 interface #41: Implements the same per-distribution S3 refactor, including the new gamma methods and dist_* dispatch interface introduced in this PR.

Possibly related PRs

• epiforecasts/distspec#32: Both PRs touch sd()/natural_params() in R/dist_spec.R; this PR changes the gamma implementations to delegate through the new distribution interface at the same functions.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately reflects the main change: introducing a per-distribution S3 interface and migrating gamma as a proof of concept.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch refactor-gamma-dispatch

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