Add Luang Prabang walkthrough vignette

Create `vignettes/evca-luang-prabang.Rmd` as a full worked case study
demonstrating all package functions using the Luang Prabang road decision.

Should cover:
- [ ] Case setup (three stakeholders, three actions)
- [ ] Normalization of incommensurable welfare units
- [ ] `compute_evca()` with equal model weights
- [ ] `compute_exclusion_costs()` + `plot_exclusion_costs()`
- [ ] `compute_evca_dirichlet()` across three prior scenarios + both plot functions
- [ ] Interpretation of each result

Also update `paper_v3.Rmd` to call the package functions for the exclusion
and Dirichlet sections rather than reimplementing them inline.