chore: bump version to 0.6.3, update README, regenerate plots

Stage 5-6 (#151-#189, 39 issues) complete on this branch.

- Project.toml -> 0.6.3
- README: Honest DiD feature line reflects the real Rambachan-Roth
  implementation (relative-magnitudes + second-difference restrictions,
  Armstrong-Kolesár FLCIs)
- docs/generate_plots.jl: two plot recipes predated the v0.6.2
  stochastic-singularity guard (n_obs > n_shocks with no measurement error
  now throws by design) — Bayesian DSGE IRF passes measurement_error=:auto,
  DSGE HD passes explicit measurement SDs
- Regenerated all 66 D3 plot HTML files: material changes in did_honest
  (real RR robust CIs + breakdown), reg_marginal_effects (intercept row
  dropped per T089 M-28), did_event_study/did_bacon (CS influence-function
  SEs, corrected Bacon weights); remaining diffs are RNG-sequence shifts
  from upstream estimator fixes in the shared generation script
