Missing taxable_roth_conversions in State Gross Income / AGI Definitions (AL, AR, NJ)

When `taxable_roth_conversions` was added as a distinct variable from `taxable_ira_distributions`, it was added to the federal gross income sources (`gov.irs.gross_income.sources`), but was omitted from several state gross income / AGI definitions that explicitly list income components (AL, AR, NJ).

This causes state models to treat Roth conversions as non-taxable at the state level. In Alabama (which allows a deduction for federal income taxes paid), this omission creates a **negative state marginal tax rate** as Roth conversions rise, because federal tax increases while Alabama gross income remains unchanged.

### Affected Files
1. **Alabama (`AL`):**
* `policyengine_us/parameters/gov/states/al/tax/income/agi/gross_income_sources.yaml`
2. **Arkansas (`AR`):**
* `policyengine_us/parameters/gov/states/ar/tax/income/gross_income/sources/individual.yaml`
* `policyengine_us/parameters/gov/states/ar/tax/income/gross_income/sources/joint.yaml`
3. **New Jersey (`NJ`):**
* `policyengine_us/parameters/gov/states/nj/tax/income/gross_income/non_negative_sources.yaml`

### Expected vs. Actual Behavior

* **Expected Behavior:**
* In **AL, AR, and NJ**, traditional IRA to Roth conversions are taxable income at the state level (just like `taxable_ira_distributions`) under Ala. Code § 40-18-14, Ark. Code § 26-51-404, and N.J.S. 54A:5-1 (Line 20a).
* State AGI / gross income should increase when `taxable_roth_conversions` increases.

* **Actual Behavior:**
* **Alabama (`AL`):** `al_agi` stays flat at $100k when Roth conversions increase from $0 to $50k, while `al_federal_income_tax_deduction` increases. This reduces `al_taxable_income` and causes state income tax to decrease, resulting in a negative marginal tax rate (-0.75% to -1.35%).
* **Arkansas (`AR`) & New Jersey (`NJ`):** State tax on Roth conversions is calculated as $0 regardless of conversion amount.

### Reproduction Scenario (Alabama Negative Marginal Rate)

```python
from policyengine_us import Simulation

sit_0 = {
'people': {'you': {'age': {2026: 45}, 'long_term_capital_gains': {2026: 100000}, 'taxable_roth_conversions': {2026: 0}}},
'tax_units': {'tax_unit': {'members': ['you']}},
'households': {'household': {'members': ['you'], 'state_code': {2026: 'AL'}}}
}
sit_1 = {
'people': {'you': {'age': {2026: 45}, 'long_term_capital_gains': {2026: 100000}, 'taxable_roth_conversions': {2026: 10000}}},
'tax_units': {'tax_unit': {'members': ['you']}},
'households': {'household': {'members': ['you'], 'state_code': {2026: 'AL'}}}
}

sim0 = Simulation(situation=sit_0)
sim1 = Simulation(situation=sit_1)

print("Roth = $0 AL Tax:", sim0.calculate('al_income_tax', 2026)[0]) # 4501.63
print("Roth = $10k AL Tax:", sim1.calculate('al_income_tax', 2026)[0]) # 4426.63 (-$75 tax on +$10k income!)
```

### Proposed Solution
Add `taxable_roth_conversions` to the parameter YAML lists for:

`gov.states.al.tax.income.agi.gross_income_sources`
`gov.states.ar.tax.income.gross_income.sources.individual`
`gov.states.ar.tax.income.gross_income.sources.joint`
`gov.states.nj.tax.income.gross_income.non_negative_sources`

Start with the four affected YAML parameter files listed in the issue and compare their income-source lists with the federal gross-income sources. Run the Alabama reproduction scenario, then verify that the Roth conversion is included for AL, both AR filing statuses, and NJ, with state income or AGI increasing as the conversion rises.

Written by the indexing model from the issue text.