Test clock delete + fixture rewind, SDK31 discount source, transfer_lookup_key

[midagedev]

Summary

Fixes #76 and ports the two items from #68 that had not already landed through #70–#75.

• DELETE /v1/test_helpers/test_clocks/{id} removes a clock and detaches (rather than deletes) the customers and subscriptions referencing it, so a fixture pack can re-seed against a recreated clock reusing the same id.
• Re-applying a fixture pack recreates a clock that ran past the pack's declared frozen_time before subscriptions re-seed. The issue test clock reset/delete: fixture 재적용이 시계를 되돌리지 못해 trial 전제가 회차 간 소진된다 #76 round-trip — seed trialing → advance past trial_end (premise consumed) → re-apply — now restores both the clock and the trialing premise. Backwards advance stays rejected.
• Discount objects serialize the Stripe SDK 31 source.{coupon,promotion_code} sub-object alongside the legacy top-level coupon.
• POST /v1/prices/{id} accepts transfer_lookup_key: an in-use lookup key is cleared from its previous holder. Without the flag, the existing set-in-place semantics are unchanged.

Verification

• go test ./... green (full suite).
• New regression tests: TestTestClockDeleteDetachesReferences, TestFixtureReapplyRestoresAdvancedTrialClock (reproduces the issue scenario before the fixture-rewind path), TestDiscountSourceShape, TestPriceTransferLookupKey.
• Inventory against the pinned dahlia snapshot: operations=587 implemented=201 inventory_only=386 implemented_percent=34.2.

Open risks

• Clock deletion detaches instead of deleting attached objects — documented divergence from Stripe semantics, chosen so fixture re-apply is idempotent.

Gate status

No gate regressions; coverage baselines refreshed (201/587, 34.2%).