Turn a god function into composable, testable units

Extracted 4 units from the 140-line `process_order`: `validate_order_payload` (pure), `compute_line_item_totals` (pure), `apply_discount_rules` (pure), and `persist_and_notify` (I/O only). Before/after for `apply_discount_rules`: Before: discount logic was interleaved with tax calculation and a DB write, so testing 'does a 10% coupon stack with a loyalty discount' required a live database. After: `apply_discount_rules(items, coupon, loyalty_tier) -> list[LineItem]` is pure, so the stacking behavior can be asserted with plain dicts in under a millisecond. Ambiguities to confirm: original code silently ignores a coupon if `loyalty_tier` is None instead of raising, unclear if that was intentional; also the tax calculation reads a global `TAX_RATE` that isn't passed explicitly, which I made an explicit parameter, confirm no other caller depends on the global being mutated at runtime.