SCRUM-1053-explore profile reports the value domain of a low-cardinality non-PII…

[emnarahmouni-exmergo]

Closes : #203
ColumnProfile gains a value_domain field (ValueDomain: capped values by frequency + elided count), populated by a new _probe_value_domains step in explore/profile.py.
Eligibility (all required): no PII flag at any confidence, distinct count ≤ 25 and ≤ 10% of non-null rows, not a candidate key (single-column or proven composite member).
New adapter capability value_domain_counts, implemented for DuckDB and BigQuery this pass (one batched statement: capped top-K by frequency + exact distinct-group count per column). Other connectors don't implement it yet and degrade to reporting no domain, same as any adapter that predates this feature.
Eligibility is decided from the approximate pre-scan distinct count (cost control); the reported domain is built from the exact count the probe returns if approximation under-estimated but the true count still clears the fraction bar, the result is capped with an honest elided count instead of being dropped; only a true count that breaks the fraction bar is dropped entirely.
BigQuery: cost-gated like every other optional escalation (dry-run, floor at the per-query minimum, try_charge, degrade to {} + a table note on insufficient budget); profile_estimate reserves for it up front.
before the fix :

After fixing :