TracingParameterNodeAtInstant never records scale/bracket parameter reads

[PavelMakarchuk]

Summary

TracingParameterNodeAtInstant.get_traced_child (tracing_parameter_node_at_instant.py:49) only calls record_parameter_access when the child is an ndarray or one of ALLOWED_PARAM_TYPES (float, int, bool, None, list):

if isinstance(child, (numpy.ndarray,) + parameters.ALLOWED_PARAM_TYPES):
    self.tracer.record_parameter_access(name, period, self.branch_name, child)

A ParameterScaleAtInstant child (SingleAmountTaxScale, MarginalRateTaxScale, …) is neither a node (so it is not re-wrapped) nor an allowed leaf, so it is returned raw and never recorded. Any formula that reads a bracket parameter through .calc() or [index] on the scale leaves no trace of it:

p = parameters(period).gov.irs.credits.ctc.amount
return qualifying_child * p.base.calc(age)      # gov.irs.credits.ctc.amount.base never recorded

Same for p.max[child_count] (EITC), p.rates.calc(income) (income tax brackets), and every state bracket schedule. Verified on policyengine-core 3.30.2 / policyengine-us 1.808.0.

Why it matters

The tracer is the only exact record of which parameters a formula reads (static analysis misses vectorised and alias-based reads). Bracket schedules are a large share of reform-relevant parameters, so a parameter dependency map built from the tracer is missing them entirely today.

Workaround

original = TracingParameterNodeAtInstant.get_traced_child
def get_traced_child(self, child, key):
    is_node = isinstance(child, (ParameterNodeAtInstant, VectorialParameterNodeAtInstant))
    is_leaf = isinstance(child, ALLOWED_PARAM_TYPES) or hasattr(child, "shape")
    if not is_node and not is_leaf:
        name = self.parameter_node_at_instant._name if not isinstance(key, str) \
            else f"{self.parameter_node_at_instant._name}.{key}"
        self.tracer.record_parameter_access(name, self.parameter_node_at_instant._instant_str, self.branch_name, None)
    return original(self, child, key)

Proposed fix

Record the access for any child that is not a parameter node, using the scale's node name and value=None (or the scale object itself). Recording at the scale node is the right granularity: rate[status].zone1 is already recorded as rate for vectorial reads, and a bracket read is best described as a read of the whole scale.

Context

Found alongside the cached-at-instant bug (#541) while building a traced parameter → variable dependency map for PolicyEngine/policyengine-app-v2#1180. Note this only reproduces once that bug is worked around, since yearly formulas otherwise record nothing at all.