clone()-based reform path breaks ParameterNode iteration, silently fails for NJ

[PavelMakarchuk]

Bug

When any reform is applied via the API, all NJ tax variables return null. NJ is the only state affected — all other 50 states + DC work correctly. NJ baseline (no reform) also works fine.

Root Cause

Two issues in country.py:

1. clone() breaks ParameterNode iteration

The API applies reforms via system.clone() + direct parameter.update() (lines 366-383) rather than Reform.from_dict(). This breaks ParameterNodeAtInstant.__iter__, which NJ's nj_gross_income formula relies on:

cats = p.loss_eligible_categories
for cat in cats:  # iterates ParameterNodeAtInstant — fails after clone()
    total += max_(add(person, period, cats[cat]), 0)

This pattern is valid policyengine-core usage but breaks under the API's clone path. It cannot be reproduced locally with Reform.from_dict().

2. Silent error swallowing (lines 447-449)

except Exception as e:
    if "axes" in household:
        pass  # Exception silently discarded

This hides the actual exception, making the bug invisible. The variable silently returns null instead of surfacing the error.

Evidence

• API trace confirms nj_gross_income (person-level) is the root failure — all downstream NJ variables cascade
• Federal variables (adjusted_gross_income, income_tax) compute correctly under the same reform
• Every other state works fine
• Cannot reproduce locally even when mimicking the clone() path
• API version: 1.592.4

Suggested Fixes

1. Investigate why clone() breaks ParameterNodeAtInstant iteration and fix the deep copy
2. At minimum, log the exception instead of silently swallowing it — the pass on line 449 should be logging.exception(e)

Workaround

PolicyEngine/policyengine-us#7743 works around this by replacing the iteration with explicit category references, but this is a band-aid — the API should support valid core patterns.