V-L1-E2: implement Temporal drift detector (first category)

[hyperpolymath]

Context

First drift category to ship: Temporal drift — versions of the same entity disagree across overlays.

What to do

After V-L1-E1 ADR pins the definition, implement tier1::constraints::detect_temporal_drift(entity_id) -> Option<DriftReport> over the verisimdb_temporal_versions table and the active Tier 2 overlays.

Acceptance

• Library function returns drift score in [0,1]
• Property test on synthetic data
• verisimiser drift subcommand wired to the real detector for this category