Add ECB HFCS Wave 2023 Belgium wealth facts

Refs #69.

Source gap

Chronicle has no official Belgium HFCS net-wealth/distribution/component
aggregate package. No ECB wave-2023 workbook bytes or checksum are present on
current main or in the inspected branches, so values must not be copied from
the rendered statistical tables.

Deterministic handoff

Retrieve the official ECB HFCS Wave 2023 statistical-tables ZIP, version 5.0
(June 2026), through the ECB entry in
FETCH-MANIFEST-BELGIUM-PUBLIC-FACTS.json:

https://www.ecb.europa.eu/home/pdf/research/hfcn/HFCS_Statistical_Tables_Wave_2023_June_2026.zip

Acceptance criteria

• Pin the unchanged outer ZIP with URL, SHA-256, size, ECB version/vintage, and
  content-addressed raw-storage pointer. List archive members deterministically
  and pin each selected native workbook member separately.
• Inspect workbook sheet names, row/column labels, units, weighting labels, and
  cells before authoring. Belgium asset/liability values refer to interview
  time during January-December 2023; income variables refer to 2022.
• Add only ECB-published Belgium net-wealth means/medians/quantiles,
  distribution shares/ratios, and component aggregates with exact source-cell
  lineage and survey_aggregate provenance.
• Do not interpolate quantiles, derive components or totals, reconcile to NBB,
  age values, or add model/target bindings.
• Run validate-package, source-cell preservation, fact-load,
  consumer-artifact, raw-facts-boundary, ruff, and git diff --check checks.

Start with FETCH-MANIFEST-BELGIUM-PUBLIC-FACTS.json and retrieve the specified ECB HFCS Wave 2023 version 5.0 ZIP. Inspect its archive members and selected workbook sheets, labels, units, weights, and source cells before adding only the permitted Belgium aggregates. Done means pinned source metadata and lineage are present, and validate-package, preservation, fact-load, consumer-artifact, boundary, ruff, and git diff --check pass.