clone() + parameter.update() breaks ParameterNodeAtInstant iteration

[MaxGhenis]

Bug

TaxBenefitSystem.clone() followed by parameter.update() breaks ParameterNodeAtInstant.__iter__ for directory-based ParameterNodes. This causes silent calculation failures when the API applies reforms.

Reproduction

The bug manifests in the API's reform path (country.py):

system = self.tax_benefit_system.clone()
for parameter_name in reform:
    parameter = get_parameter(system.parameters, parameter_name)
    parameter.update(start=..., stop=..., value=...)

# Later, a formula iterates a ParameterNode:
cats = p.loss_eligible_categories  # directory-based ParameterNode
for cat in cats:  # fails after clone() + update()
    total += max_(add(person, period, cats[cat]), 0)

Cannot reproduce locally with Reform.from_dict() or even with manual clone() + update() — the failure is specific to the deployed API environment. The exact mechanism is unclear.

Impact

• NJ state tax calculations silently returned null for all reform scenarios (fixed in Fix NJ gross income formula that breaks API reform calculations policyengine-us#7743 by replacing iteration with explicit attribute access)
• Any formula iterating a directory-based ParameterNode would be affected under reforms
• Combined with silent exception swallowing in the API (clone()-based reform path breaks ParameterNode iteration, silently fails for NJ policyengine-api#3322, fix in Log exceptions instead of silently swallowing them in calculate policyengine-api#3323), this was invisible in production

Expected behavior

ParameterNodeAtInstant.__iter__ should work correctly after clone() + parameter.update(), the same as it does with Reform.from_dict().

Context

• API version: 1.592.4
• Only directory-based ParameterNodes are affected (not unit: list parameter files)
• Reform.from_dict() works fine — only the clone path breaks