Record folded-booking line 381 dayCount equivalent

TODO.md

@@ -1397,19 +1397,9 @@ reorganisation. Starting point: move the two phone-index describe blocks into
 `storedPhoneIndex` helpers in whichever file needs them (or lift to `#test-utils`
 if both do). The webhook test file drops to ~380 lines.
 
-## Mutation survivor in `src/features/api/folded-booking.ts` (direct tests)
+## Mutation coverage of `src/features/api/folded-booking.ts` (direct tests)
 
 Direct tests at `test/features/api/folded-booking.test.ts` and
-`test/features/api/folded-booking/parent-booking.test.ts` kill all but one mutant on the unchanged `folded-booking.ts`;
-four equivalents (lines 87, 118, 176, 301) are recorded in
-`scripts/mutation/equivalent-mutants.txt`. One survivor remains, noted here
-because it is a deliberate non-kill (testing it would assert an
-implementation detail, not a user-visible contract):
-
-- `src/features/api/folded-booking.ts:381:17  1 → 0` —
-  `dayCount: 1` in the `FoldBase` `processParentApiBooking` builds. The fold
-  computes its own `fold.dayCount` (e.g. `3` for a customisable child) which
-  the intent carries, so the base value does not surface in any observable
-  output of the parent flow. Killing it would require asserting on the
-  intermediate `parentResolvedDuration(parent.listing, base.dayCount)` value,
-  which is an implementation detail rather than a user-visible contract.
+`test/features/api/folded-booking/parent-booking.test.ts` kill every non-equivalent mutant on the unchanged `folded-booking.ts`.
+Five equivalents (lines 87, 118, 176, 301, 381) are recorded in
+`scripts/mutation/equivalent-mutants.txt` with proofs — no unsuppressed survivors remain.

scripts/mutation/equivalent-mutants.txt

@@ -908,3 +908,4 @@ src/features/api/folded-booking.ts:87:42  ?? → ||   # childrenByParentId is Ma
 src/features/api/folded-booking.ts:118:31  ?? → ||   # qtyByChild accumulates ApiQuantitySchema values (positive integers >= 1) or is unset; it is never 0 or NaN for validated inputs, so ?? 0 and || 0 agree
 src/features/api/folded-booking.ts:176:53  1 → 0   # the thankYouUrl guard checks fold.listings.length > 1 only when parentThankYouUrl is truthy (parent flow), which always folds parent + >= 1 child (length >= 2); package callers pass no parentThankYouUrl, so the && short-circuits and the check never runs
 src/features/api/folded-booking.ts:301:70  0 → 1   # feeSubtotal flows into priceCheckout → owedOrderForLedger, which drops all extras and sets total to 0; bookingFactsFromOrder reads extras (empty) → bookingFee: 0 regardless. The feeSubtotal value never reaches any observable output.
+src/features/api/folded-booking.ts:381:17  1 → 0   # base.dayCount is read only by parentResolvedDuration → childDaysFromParent(parent, dayCount, 1), which uses its first argument (dayCount) only in the parent.customisable_days branch; processParentApiBooking rejects customisable parents at line 335 before reaching the fold, so that branch is unreachable. For daily parents the function returns clampDurationDays(parent.duration_days), and for standard parents it returns 1 (the standardValue) — neither reads dayCount, so 1 and 0 produce the same duration and thus the same fold output for every reachable input.