Add IRD taxable-income and wage statistics for New Zealand tax year 2024

Part of NZ source-expansion issue #176; this bounded task covers source families I1 and I2 only. Add publisher-backed TY2024 taxable-income band counts and totals, age-by-income counts, and wage/salary band counts and totals. Pin the official taxable-income September 2025 workbook and the revised August 2026 wage workbook, with source-cell lineage, explicit April 2023–March 2024 reference periods, immutable raw/nz provenance, deterministic gates, and source-fidelity/boundary review. The wage source is now revised; name its actual August 2026 publication vintage rather than relabeling it as a 2025 release. Preserve publisher labels, low-income and unknown-age categories, negative tax-on-taxable-income amounts, and separately published totals. No reconciliation, uprating, target activation, or policy-model outputs belong in this change. This task does not close the full #176 fifteen-package inventory.

Start by reading source-expansion issue #176 and its fifteen-package inventory, then identify the existing patterns for source families I1 and I2. Use the official taxable-income September 2025 workbook and revised August 2026 wage workbook as the source references. Done means TY2024 statistics, publisher labels, periods, provenance, source-cell lineage, deterministic gates, and boundary review are captured without reconciliation or policy-model outputs.