test: stream-driven tests for card view-models

[Zeroupper]

Problem

(verified) The samplingTable double-count bug (lib/view_models/cards/measurements_data_model.dart:22-29) was invisible precisely because no test pushes synthetic measurements through these view-models; test/heart_rate_data_model_test.dart already shows the pattern (mocked controller + streams).

Proposed fix

Reuse that harness for steps/activity/mobility/measurements after the leak-EPIC fixes; assert single-subscription behavior on re-init.

Acceptance criteria

• Each card view-model has a test feeding >= 2 measurements and asserting counts and re-init behavior

Effort: M · Priority: P2 · Change risk: None