Claude Haiku 5.5 (effort default (no --effort flag) · thinking on) on Fix a flaky test suite caused by set ordering, ignored date parameter and global random state - without touching the tests

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  "result": "`billing.py` is now deterministic. All three test functions pass under `PYTHONHASHSEED` 0, 1, 2, 42 and 999. I did not modify `tests/test_billing.py`.\n\nThere were three causes of the flakiness:\n\n- **`today` was always overwritten** with `datetime.date.today()`, so any test passing a fixed date failed on every day that wasn't that date. It now defaults only when `None` is passed.\n- **Invoice numbering came from iterating a `set`**, whose order depends on the hash seed. Customers are now sorted by `(name.casefold(), name)`, so the order is case-insensitive with exact-name tie-breaking, and numbering follows that order.\n- **The audit used the global `random.sample`**, so results depended on global state. It now uses a private `random.Random` seeded from a SHA-256 digest of the batch's date, `audit_size`, and the invoice contents. The digest is stable across processes and hash seeds, and the global RNG is never touched.\n\nI also made a few small changes:\n\n- Totals are accumulated in a single pass over `orders`, not one scan per customer.\n- Due dates are computed once rather than per invoice.\n- The signature is unchanged.\n\n**Checks I ran:**\n- pytest isn't installed here, so I called the three test functions directly.\n- Global `random` state is unchanged after calling `build_batch`.\n- Across the 12 months of 2026 with 30 customers and `audit_size=5`, all 12 samples differ, and they are not just the first N invoices.\n- `audit_size` larger than the number of invoices is clamped.\n- Mixed-case names sort as `_x, A, a, B, b`.\n\nThis directory is not a git repository, so I did not commit anything.",
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