The eight assertions

SELECT
  order_id,
  line_number,
  COUNT(*)                        AS rows_at_grain,
  COUNT(DISTINCT order_line_sk)   AS distinct_sks
FROM gold.fct_order_line
GROUP BY order_id, line_number
HAVING COUNT(*) > 1

SELECT
  f.order_line_sk,
  f.customer_sk,
  f.order_ts
FROM gold.fct_order_line f
LEFT JOIN gold.dim_customer d
       ON d.customer_sk = f.customer_sk
      AND f.order_ts >= d._tech_valid_from
      AND (d._tech_valid_to IS NULL OR f.order_ts < d._tech_valid_to)
-- IS DISTINCT FROM, not <>: a NULL customer_sk must be REPORTED, and
-- f.customer_sk <> 'UNKNOWN' is NULL on exactly those rows.
WHERE f.customer_sk IS DISTINCT FROM 'UNKNOWN'
  AND d.customer_sk IS NULL

SELECT
  country,
  COUNT(*) AS affected_rows,
  MIN(customer_id) AS example_customer
FROM gold.dim_customer
WHERE country IS NULL
   OR country NOT IN ('DE', 'AT', 'CH', 'NL')
GROUP BY country

-- Intervals are [_tech_valid_from, _tech_valid_to) — end is EXCLUSIVE.
-- The asymmetric join reports each offending pair once, not twice.
SELECT
  a.customer_id,
  a._tech_valid_from AS earlier_from,
  a._tech_valid_to   AS earlier_to,
  b._tech_valid_from AS later_from
FROM gold.dim_customer a
JOIN gold.dim_customer b
  ON  b.customer_id       = a.customer_id
  AND b._tech_valid_from  > a._tech_valid_from
WHERE a._tech_valid_to IS NULL
   OR b._tech_valid_from < a._tech_valid_to

SELECT
  customer_id,
  COUNT_IF(_tech_is_current)                            AS flagged_current,
  COUNT_IF(_tech_valid_to IS NULL)                      AS open_intervals,
  COUNT_IF(_tech_is_current AND _tech_valid_to IS NULL) AS both_on_same_row
FROM gold.dim_customer
GROUP BY customer_id
HAVING COUNT_IF(_tech_is_current)                            <> 1
    OR COUNT_IF(_tech_valid_to IS NULL)                      <> 1
    OR COUNT_IF(_tech_is_current AND _tech_valid_to IS NULL) <> 1

SELECT
  order_line_sk,
  currency,
  fx_rate,
  quantity,
  unit_price,
  amount_eur,
  status,
  ROUND(quantity * unit_price * fx_rate, 2) AS expected_eur
FROM gold.fct_order_line
WHERE amount_eur IS DISTINCT FROM ROUND(quantity * unit_price * fx_rate, 2)
   OR (currency = 'EUR' AND fx_rate IS DISTINCT FROM 1)
   OR (status IS DISTINCT FROM 'CANCELLED' AND COALESCE(amount_eur, 0) <= 0)
   OR currency IS NULL
   OR currency NOT IN ('EUR', 'CHF')

WITH line AS (
  -- LEFT JOIN to the header, for the same reason as the rate join below: an
  -- order line whose header has not landed yet (late CDC on bronze.sap_vbak)
  -- would be deleted from the SILVER side by an inner join, and if gold's build
  -- joins the same way it is deleted from BOTH sides equally — delta 0,00 on a
  -- month that is short a whole order.
  SELECT
    l.order_id,
    l.line_number,
    l.quantity,
    l.unit_price,
    l.currency,
    l.status,
    -- the header carries the authoritative order date; fall back to the line's
    -- own timestamp so a headerless line still lands in a month. If it has
    -- neither, its month is NULL — a bucket the FULL OUTER JOIN below keeps
    -- visible instead of deleting.
    COALESCE(o.order_ts, l.order_ts)  AS order_ts,
    o.order_id                        AS header_order_id
  FROM silver.sales_order_line l
  LEFT JOIN silver.sales_order o ON o.order_id = l.order_id
),
silver_month AS (
  SELECT
    DATE_TRUNC('MONTH', x.order_ts)                       AS month,
    -- EUR is the base currency and is absent from the ECB file, so it is 1 by
    -- definition. Rule 2 rounds PER LINE; rounding the sum drifts by cents.
    SUM(ROUND(x.quantity * x.unit_price *
              CASE WHEN x.currency = 'EUR' THEN CAST(1 AS DECIMAL(18,6))
                   ELSE r.fx_rate END, 2))                AS silver_eur,
    -- a missing rate makes BOTH sides smaller by the same amount, so the delta
    -- stays zero. This column is the only place that defect is visible.
    COUNT_IF(x.currency IS DISTINCT FROM 'EUR'
             AND r.fx_rate IS NULL)                       AS lines_without_rate,
    -- and the same argument for the header. Same shape, same silence.
    COUNT_IF(x.header_order_id IS NULL)                   AS lines_without_header
  FROM line x
  LEFT JOIN silver.fx_rate r ON r.currency  = x.currency
                            AND r.rate_date = DATE(x.order_ts)
  WHERE x.status IS DISTINCT FROM 'CANCELLED'
  GROUP BY 1
),
gold_month AS (
  SELECT
    DATE_TRUNC('MONTH', order_ts) AS month,
    SUM(amount_eur)               AS gold_eur
  FROM gold.fct_order_line
  WHERE status IS DISTINCT FROM 'CANCELLED'
  GROUP BY 1
)
SELECT
  COALESCE(s.month, g.month)                            AS month,
  s.silver_eur,
  g.gold_eur,
  COALESCE(g.gold_eur, 0) - COALESCE(s.silver_eur, 0)   AS delta_eur,
  COALESCE(s.lines_without_rate, 0)                     AS lines_without_rate,
  COALESCE(s.lines_without_header, 0)                   AS lines_without_header
FROM silver_month s
FULL OUTER JOIN gold_month g ON g.month = s.month
WHERE ABS(COALESCE(g.gold_eur, 0) - COALESCE(s.silver_eur, 0)) > 0.01
   OR COALESCE(s.lines_without_rate, 0)   > 0
   OR COALESCE(s.lines_without_header, 0) > 0