Harmonize state_standard_deduction and state_itemized_deductions coverage

[PavelMakarchuk]

Background

PR #8213 fixed the user-visible bug in tax_unit_itemizes (it now falls back to federal-level deduction comparison when federal tax is tied between the itemize/standard branches). But it explicitly left the underlying helper asymmetry in place:

• state_standard_deduction.py covers 8 states: CT, ID, MO, ND, NM, SC, UT, CO.
• state_itemized_deductions.py covers only 5 states: CT, GA, ND, SC, UT. For everyone else it sums state-specific line items, which for many states evaluates to $0.

The helpers are still exposed via TAXSIM v34 ("Standard Deduction") and v35 ("Itemized Deductions") in policyengine-taxsim's variable_mappings.yaml. So a TAXSIM consumer querying a CO household still sees state_standard_deduction = $31,500 and state_itemized_deductions = $0, which is misleading.

Proposed work

Decide on consistent semantics, then apply across all states:

1. Pick a contract. Options:
   • "Returns the actual amount the household would deduct on the state return." Requires per-state research; many states have hybrid rules (separate caps, add-backs, federal-conformity dates).
   • "Returns NaN or raises for unmapped states." Conservative — surfaces gaps rather than silently returning $0.
2. Per-state audit. For each state with an income tax (≈42 states + DC):
   • Standard deduction: federal-conforming? Own amount? None?
   • Itemized deductions: federal-conforming? Own subset (e.g. SALT caps differ)? None allowed?
3. Update both helpers to match the chosen contract.
4. Update TAXSIM mapping if the contract changes semantics.

Out of scope

• The tax_unit_itemizes decision logic (already fixed in Fix tax_unit_itemizes tied-federal-tax fallback (#8210) #8213).

References

• PR Fix tax_unit_itemizes tied-federal-tax fallback (#8210) #8213 (the targeted fix)
• TAXSIM tickets driven by this asymmetry: #870 (CO), #869 (VT), #868 (RI), #854 (LA)
• policyengine-taxsim's variable_mappings.yaml for v34/v35 consumers