pvstackr 0.2.0

The first hardening release. It makes stored objects portable across R
versions and machines, opens the bundled brms adapter as a public three-function
interface, and tightens what a fit is allowed to retain when it is blocked.

Portability (changes hash values)

• Content hashes and validation stamps no longer depend on the R version that
  wrote them. Both hashed the whole serialization stream, whose fourteen-byte
  header records the writing R version. Content hashes computed by earlier
  releases will not match the ones this release derives, so a stored
  design_hash from 0.1.x is expected to differ.
• A saved fit no longer refuses its own accessors after an R upgrade.
• pv_revalidate_brr_target() no longer requires a legacy target's weighted
  least-squares estimates to match bit-for-bit. Those last bits differ between
  BLAS implementations, so a target built on one machine could not be
  revalidated on another. Structure, declared inputs, and the input-derived
  design hash are still compared exactly; the estimates go through the
  package's numeric policy (absolute 1e-12, relative 1e-10). A difference
  large enough to matter still fails closed.

Backend

• The bundled brms adapter is public: pv_backend_brms_fit_function(),
  pv_backend_brms_draws_function(), and pv_backend_brms_sampler_diagnostics().
  Injecting all three runs the same code the bundled backend runs; any one can
  be replaced to attach a different engine. The routes differ in recorded
  provenance, and a reportable injected fit needs all three.
• pv_fit(method = "stack_direct") with pv_control(backend = "brms") now runs
  without an injected adapter pair.
• A population-level prior (class = "b" with no coef) is expanded onto the
  individual slope coefficients instead of being refused.

Fail-closed behaviour

• Final fits carry an exact validation record with a deep tier and a cheap
  rehash tier.
• Blocked fits use a generic retention firewall: heavy payloads are absent, and
  only canonical evidence is retained.
• stack_psis separates weight input route, Pareto-k source, and smoothing
  provenance; supplied weights enter the reportable route only when the caller
  declares an external producer and version.
• pv_migrate_legacy_psis_fit() gives historical PSIS results an
  inspection-only migration path.

See NEWS.md for the
full list, and the package website for
documentation.