Compute Baseline

Workflow

Per week:
- [ ] Step 1: Load last 4 weekly CSVs from corpus/stats/
- [ ] Step 2: For each metric (open_rate, click_rate, views_per_send, weekly_sub_delta):
    - Compute 4-week median
    - Compute trimmed median (drop 1 outlier)
    - Compute IQR
- [ ] Step 3: z-score = (current - median) / IQR
- [ ] Step 4: If <4 weeks in history, return baseline: not-yet-established
- [ ] Step 5: Emit baseline object per metric with confidence flag

Baseline object schema