powerbrmsINLA 1.2.0

Major additions

New compute_assurance() function for unconditional Bayesian assurance (O’Hagan & Stevens, 2001) computed as a weighted average of conditional power over a design prior on the effect size.
New assurance_prior_weights() convenience wrapper for constructing normalised design-prior weights (normal, uniform, beta) over an effect grid.
New decide_sample_size() function with both assurance mode (design prior) and conditional mode for recommending sample sizes from simulation output.
New validate_inla_vs_brms() function for spot-checking INLA posterior estimates against brms/Stan.
Print methods for brms_inla_power, powerbrmsINLA_assurance, and powerbrmsINLA_sample_size objects.

Added plot_assurance_curve() and plot_assurance_with_robustness() for unconditional assurance visualisation.
Added plot_bf_assurance_curve_smooth(), plot_bf_assurance_curve(), plot_bf_expected_evidence(), and plot_bf_heatmap() for Bayes factor visualisation.
Added plot_decision_assurance_curve(), plot_decision_threshold_contour(), and add_decision_overlay() for decision-rule visualisation.
Added plot_design_prior() for visualising design priors.
Added plot_interaction_surface() for multi-effect grid visualisation.
Added plot_power_contour(), plot_power_heatmap(), and plot_power_assurance_overlay() for conditional power visualisation.
Added plot_precision_assurance_curve() and plot_precision_fan_chart().

brms_inla_power() now supports multi-effect grids (data.frame effect_grid), brms-to-INLA prior translation with full audit trail, marginal-likelihood Bayes factors (bf_method = "marglik"), and automatic INLA thread detection.
brms_inla_power_sequential() rewritten with multi-effect support and prior translation.
brms_inla_power_two_stage() now uses the modernised engine internally.

Removed duplicate internal function definitions (.to_inla_family(), .scale_fill_viridis_discrete()).
Added requireNamespace("MASS") guard for negative binomial data generation.
Expanded .Rbuildignore to exclude .claude/, .DS_Store, .Rcheck/, and .tar.gz artefacts.
New test suites for assurance computation, decision helpers, prior bridge, and plotting functions.

error_sd and group_sd now accept distributional specifications (halfnormal, lognormal, uniform) for variance-uncertainty integration; new validate_sd_spec() helper exported.
Added validation test suite (test-validation-classical.R, test-validation-bayesassurance.R) and accompanying vignette benchmarking against power.t.test() and bayesassurance::assurance_nd_na().
CRAN housekeeping: exclude .github, LICENSE.md, and cran-comments.md from the source tarball via .Rbuildignore.