Add official NZ WFF and IWTC source facts

[MaxGhenis]

Summary

Implements package 8 of the canonical New Zealand wave-1 inventory in #176:
ird/working_for_families_statistics_sept_2025. This targets main after #208's
country-storage and period conventions merged as 10597ae. It does not close #176 or claim that
Microcosm calibration or reform scoring is live.

• Pin Inland Revenue's original September 2025 workbook, unchanged (71,211
  bytes; SHA-256 95ae66f4d44f3f47ea3daa006328b22f061a163cf7e31b487342cde649390833).
• Emit 330 administrative source facts for the 2024 NZ tax year: national
  WFF/FTC/MFTC/IWTC/BSTC recipient-family counts and aggregate entitlements,
  supported children and family sizes, and the complete 39-row, 8-measure
  published family-income table.
• Preserve country NZ and tax-year coverage 2023-04-01 through 2024-03-31.
  Counts describe non-zero administrative entitlements, not all eligible
  families or cash paid during a financial year.
• Keep source income labels, unknown-income rows, publisher totals, and
  independently rounded amounts. Do not infer interval boundaries, impute a
  residual, reconcile tables, age values, or build solver targets. The explicit
  rounding-adjustment row is parsed and pinned in tests, not emitted as a
  population fact. Obsolete blank credits and the average-entitlement table
  are outside this package's emitted count-and-total-amount scope.
• Publish the single workbook under its immutable raw/nz/ird/... key and
  verify its checksum after downloading it again. The R2 manifest update is
  isolated in commit 25f5aba after the source pin in c9d6eae.

Verification

• Post-review source/storage/core/consumer/import/schema tests: 112 passed.
• The income-definition correction retains entitlement-time-weighted assessed family scheme income and the Work and Income non-filer fallback to individual tax-return income excluding partners, in every consumer-facing income measure note.
• uv run ruff check and git diff --check: passed.
• build-suite for the WFF package: all 12 source acceptance checks passed;
  4,910 source cells, 330 source records/facts/consumer rows, 100% cell lineage,
  one checksum-pinned raw artifact with an R2 link, zero acceptance errors.
• Single-source build-bundle --year 2023: passed with 330 facts, tax year
  still 2024, one source/geography, zero duplicate keys/errors/warnings.
• Full default-bundle expectations updated by the measured WFF-only delta:
  +330 facts, +1 source/package/geography, +329 family facts and +1 person fact.
  The full all-country bundle test is left to CI to avoid repeating its long
  CPU-intensive build on the user's laptop; no full-suite-green claim here.
• Axiom concept validation was not requested: these are IRD source-label
  concepts, not model-variable bindings. The suite records that unchecked
  alignment warning explicitly.

Downstream identifiers

Alias: ird-working-for-families-statistics-sept-2025; publisher: ird.

• WFF families: ird_wff_statistics.ty2024.recipient_families.all.wff_recipient_families
• IWTC families: ird_wff_statistics.ty2024.recipient_families.all.iwtc_recipient_families

These exact scalar IDs distinguish the national totals from the 39 income-table
rows sharing each source measure. Consumer measurement and any receipt/take-up
mapping remain in Microcosm.

Review gates

Independent ledger-source-fidelity and ledger-boundary reviews PASS at 98465d5. The source, checklist, and regression-test files are unchanged after merging main at eb6ca1b. #208 is merged; passing CI for the current head remains required before merge. The UK copy-only migration remainder in #175 is separate and not changed here.