Add Oregon kicker credit analysis notebook

Create a notebook to analyze the impact of Oregon's kicker credit for 2025.

The notebook should:
- Compare baseline (no kicker) vs current law (with kicker)
- Show fiscal impact on state revenue
- Show distributional impact by income decile
- Show poverty impact
- Handle the `or_tax_before_credits_in_prior_year` input variable by injecting current-year tax as a proxy