Add comprehensive E2E acceptance tests for stats progression:
- Test #1: Baseline eggs/feed/costs with 13 ducks
- Test #2: Mixed group proration with juveniles
- Test #3: Split flock per-location stats
- Test #4: Backdated eggs use historical roster
- Test #5: Event edit with revision tracking
Tests use cumulative fixtures building on each other.
Note: Some cost_per_egg_layers values differ from spec due to
integer bird-day truncation in implementation vs fractional
in spec calculations.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
