Agent B: add a receipt preview for carts

[RollNuts]

Adds orders/receipt.py with a preview that calls calculate_total(items) exactly as main defines it today. Green on its own — but if the currency PR lands first, this call shape no longer matches.

[chatgpt-codex-connector]

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[veripsa-core]

Veripsa — heading to main

❓ Unknown — not enough signal to call this clear

This PR reserves: orders/receipt.py, tests/test_receipt.py

New paths in this PR — coupling will be computable after merge. These paths are NEW in this PR, so there is no main-side graph to compare against yet (expected, not a warning): orders/receipt.py, tests/test_receipt.py. Veripsa will compute their coupling once this PR lands.

_{Veripsa records what is heading to main and who reserved what — it does not assert correctness. Advisory by default; your branch-protection policy decides what blocks.}

[RollNuts]

Closing — superseded by the fresh canonical collision pair #13 / #14 (2026-07-20)

Part of the earlier demo round. The current canonical demo is the clean two-PR collision pair #13 (Agent A) / #14 (Agent B) on orders/pricing.py. This PR (orders/receipt.py) is kept out of active demo traffic to keep the demo to exactly two PRs.

Historical evidence preserved (not deleted): this PR and its Veripsa output — Veripsa — Unknown (neutral) — remain in-thread: #8 (comment)