Reform.from_dict applies bare-date period keys for a single instant, silently (and its bare except swallows update errors)

[MaxGhenis]

Problem

Reform.from_dict (policyengine_core/reforms/reform.py) has two footguns in its period-key handling:

for period, value in period_values.items():
    try:
        period = period_(period)
        parameter = parameter.update(period=period, value=value)
    except:
        if "." in period:
            start, stop = period.split(".")
            ...

1. Bare date keys apply for a single instant, silently. {"param": {"2026-01-01": 0.39}} parses via period_("2026-01-01") and updates the parameter only at that instant — every later year reads the baseline value again. No warning. Callers almost never mean this: the natural reading (and what downstream wrappers document) is "from this date onward." This produced silently-wrong multi-year revenue estimates downstream (Open-ended reforms apply only to the start year in microsimulation (bare-date period keys) policyengine.py#453: a 10-year window scored the reform's start year correctly and exactly $0.00 for the nine later years).

2. The bare except: swallows everything from the first branch — not just period-parse failures but errors raised by parameter.update itself (bad value type, immutable parameter, etc.), which then re-enter the range-split path and surface as an unrelated secondary error (or, if the key happens to contain a ., silently take the wrong code path).

Repro (~30 s, no data)

from policyengine_us import CountryTaxBenefitSystem
from policyengine_core.reforms import Reform

def probe(period_key):
    sys_ = CountryTaxBenefitSystem()
    R = Reform.from_dict(
        {"gov.irs.income.bracket.rates.7": {period_key: 0.39}},
        country_id="policyengine_us",
    )
    reformed = R(sys_)
    p = reformed.parameters.get_child("gov.irs.income.bracket.rates.7")
    return [float(p(f"{y}-01-01")) for y in (2026, 2027, 2030, 2035)]

print(probe("2026-01-01"))            # [0.39, 0.37, 0.37, 0.37]  ← silent truncation
print(probe("2026-01-01.2100-12-31")) # [0.39, 0.39, 0.39, 0.39]

Suggested fixes

• Preferred: treat a bare date key as open-ended (value applies from that instant onward), matching the scalar branch's forever semantics (period="year:2000:100") and the intuitive reading. If changing semantics is too risky for existing callers, emit a loud warning when a bare date is applied as a single instant.
• Independently: narrow except: to the period-parse exception so parameter.update errors surface directly.

Downstream, policyengine.py now serialises explicit start.stop ranges for all reforms (PolicyEngine/policyengine.py#454), so this is hardening for direct core users and other wrappers.