Round AGI to whole dollars before the Arkansas low income table lookup #9648

Problem

The Arkansas instructions say "Round all amounts to the nearest dollar" (2024 AR1000F/AR1000NR instructions, PDF page 12: "$10,897.50, round to $10,898 … $10,897.49, round to $10,897"). The same text is in the 2025 booklet.

ar_low_income_tax_joint passes unrounded AGI to each low income table's calc(..., right=True), where each threshold is the upper bound of the previous published row. So any fractional AGI just past a row edge gets the next row's tax. Example on main (3144e5a): head of household with 0–1 dependents in 2024, $20,282.25 gets $65, where the rounded $20,282 falls in the $0 row. This affects every row edge of every table and year.

Fix: round the lookup AGI half-up to whole dollars (not banker's rounding), then add YAML cases on both sides of an edge, for example $20,282.49 → $0 and $20,282.50 → $65 for 2024 HoH 0–1 dependents.

Related cleanup (from the #9599 reviews; verify each before changing)

• Stale booklet anchors. These use printed pages instead of PDF pages. The tables are on PDF pages 29/30 in 2023 and 28/29 in 2024.
  • single.yaml: 2023 #page=25, 2024 #page=24
  • head_of_household/no_or_one_dependent.yaml: 2023 #page=25
  • surviving_spouse/no_or_one_dependent.yaml: 2023 #page=25, 2024 #page=24
  • joint/no_or_one_dependent.yaml and joint/two_or_more_dependents.yaml: 2023 #page=26, 2024 #page=25
  • Fix duplicate 2024 Arkansas low-income table threshold and guard scale thresholds #9599 fixes the two two_or_more_dependents files for HoH and surviving spouse.
• ar_low_income_tax_joint's label says "when married couples are filing separately". It actually computes the combined-return table tax (separate filers get np.inf). Its reference cites only the 2022 booklet.
• Invalid reference URLs. ar_income_tax_before_non_refundable_credits_{indiv,joint,unit}.py concatenate two reference URLs with no comma, which makes one invalid URL.
• Test gaps in ar_low_income_tax_joint.yaml:
  • no case with exactly 1 dependent (the <= 1 / > 1 boundary);
  • no surviving spouse 0–1 case;
  • no 2024 joint 2+ first-edge case (28,953 → 0; 28,954 → 108);
  • Cases 1 and 5 are named "1 dependent" but set tax_unit_dependents: 2.

Axiom parity for the tables themselves is tracked in TheAxiomFoundation/rulespec-us#1425.