Conformance coverage: loose billing_cycle_anchor allowance, and test-clock cassettes for renewal and discount-window behaviour #22

What happens

• The anchor allowance is too loose. The **.billing_cycle_anchor allowed difference in tests/conformance/allowed_differences.py (~1493) accepts any pair of integers. That is why replay never caught the stale anchor after trial_end: "now" (cassette 12 seq 29 and seq 39, fixed in Fix renewal periods, pending-item sweep, repeating coupons and canceled_at (#12–#15) #17), and it would hide a regression of that fix. Other blanket timestamp allowances may have the same problem.
• Some behaviour has no recording. Behaviour that only happens when time passes isn't covered by any cassette:
  • whether an invoice minted exactly at a discount's end is discounted (the world says no);
  • renewal line periods and the invoice's look-back period;
  • the day clamp on a repeating coupon's end;
  • day 29–31 renewals;
  • Stripe's 1-hour draft before a renewal finalizes.

Why it matters

These rulings rest on documentation and probes, not recordings. The Stripe MCP can't advance test clocks, so they can't be checked live.

Suggested next steps

• Change the anchor allowance to "equal, or equal to the replayed call's clock". This needs a comparator that knows the request time, or a per-step predicate.
• Add a tools_dev.record scenario that uses test clocks: subscribe with a 3-month repeating coupon, advance through four renewals, and record the invoices. A test-mode key is required.