dist.structure 0.5.0

Initial CRAN submission.

New exports

validate_dist_structure(x): helper for subclass implementors. Checks the class chain, presence of ncomponents() / component(), and (via getS3method lookup) whether at least one of phi.X or min_paths.X is registered. Fails fast at construction time instead of at first method dispatch.
min_paths.dist_structure: default derived by enumeration from phi. Closes a protocol gap: the contract said “provide phi or min_paths” but only the phi-from-paths direction had a default; users providing only phi previously hit “no applicable method” on min_paths() and any default that depended on it (min_cuts, system_signature, reliability, …).
surv.series_dist: product of component survival functions. Avoids the 2^m reliability-polynomial enumeration in the dist_structure default for series systems whose components are not in a closed-form family.
surv.parallel_dist: 1 - prod(component CDFs). Same rationale as surv.series_dist.
hazard.wei_series: closed-form additive Weibull hazard sum_j (k_j/s_j) * (t/s_j)^(k_j - 1).
hazard.wei_kofn, hazard.exp_kofn: composition of the existing closed-form density() and surv().
component.dual_of_system: delegates to the underlying original. User-defined dist_structure subclasses that route dual() through the lazy-wrapper path now compose with surv(), sampler(), and other defaults.

Added seven vignettes covering core workflows:
- Getting started: five-minute tour of the package.
- Coherent systems: phi, min_paths, min_cuts, signature, reliability polynomial, dual, critical states; series, parallel, k-of-n, bridge, and custom coherent systems.
- Distribution interface: dist_structure inherits from dist; default composition of surv/cdf/sampler via the reliability polynomial; when to choose specializations over the general path.
- Importance measures: structural, Birnbaum reliability, criticality, Vesely-Fussell; worked through the bridge network.
- Composition and substitution: substitute_component and compose_systems for hierarchical construction.
- Non-coherent systems: cold_standby_dist and its deliberate exclusion from the dist_structure protocol.
- Implementing a dist_structure subclass: 4-step recipe with a worked alarm-system example.
Added README.md with quick-tour and ecosystem context.
Added pkgdown configuration (_pkgdown.yml).
Per-method @return blocks across dist_structure and the closed-form family topics.
kofn_dist: explicit :G vs :F convention note with @seealso to order_statistic.
bridge_dist: Barlow-Proschan reference and component-role description.

hazard.exp_series is now properly registered as an S3 method rather than being exported as a regular function.
system_signature guards against always-zero phi (signature is undefined for non-functioning systems; previously silently returned (1, 0, ..., 0)).
mean.cold_standby_dist falls back to Monte Carlo when component means silently propagate NA instead of returning NA.
coherent_dist (and inherited by all topology shortcuts) rejects NULL or non-dist components with a clear message instead of letting the error surface deep in default-method dispatch.

Split R/defaults.R: shared numeric helpers move to R/internal-utils.R; defaults.R now contains only the S3 default methods (165 lines down from 318).
Removed seven redundant cdf overrides; cdf.dist_structure default produces an identical closure via dispatch.
exp_parallel: flipped so surv is primary, matching the family convention.
Removed redundant min_paths.kofn_dist (the parent coherent_dist already caches combn output as $min_paths).
Test seeds migrated to withr::local_seed() (auto-restore on test_that exit); withr added to Suggests.

Added closed-form density.exp_kofn and density.wei_kofn methods via the critical-state subset enumeration formula f_sys(t) = sum_j f_j(t) * P(j critical at t). Required for downstream packages (kofn) that want to compute exact loglik contributions without duplicating the math.

Added closed-form density.exp_series and hazard.exp_series methods so downstream packages can delegate via algebraic.dist::density() / hazard() rather than reimplementing.

gamma_series(shapes, rates): closed-form series of Gamma components.
lognormal_series(meanlogs, sdlogs): closed-form series of Lognormal components.
cold_standby_dist(components): non-coherent system; deliberately does NOT inherit dist_structure. Provides exact mean (sum of component means) and sampler; Monte Carlo surv/cdf with per-closure sample caching for determinism.

as_dist_structure(x): coercion. dist -> 1-component series_dist; dist_structure returned unchanged.

Added make_component_samplers and series_surv_product helpers (now shared across the exp/wei series and k-of-n specializations).

Importance measures: birnbaum_importance, criticality_importance, vesely_fussell_importance.
Compositional operations: substitute_component, compose_systems.

Critical bug in minimize_sets: equal-length duplicate sets were not being removed, causing min_cuts to return duplicated cut sets for k-of-n and bridge topologies. Fixed by deduplicating input before subset-minimality checks.
criticality_importance now validates j bounds (matches other importance measures).

Added system_signature.dist_structure default via m! permutation enumeration (warns for m > 9).

Initial release. Protocol (generics + virtual base class), topology defaults (phi, min_cuts via Berge transversal, critical_states, system_lifetime, system_censoring, is_coherent, structural_importance, reliability), dist defaults (surv, cdf, sampler via component composition), reference implementations (coherent_dist, series/parallel/kofn/bridge/consecutive_k_dist), iid convenience constructors (min_iid, max_iid, order_statistic), and closed-form series specializations (exp_series, wei_series, wei_homogeneous_series).