test: stream-driven tests for card view-models

## 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