…ure (v1.2.0)

Opt-in via HYPERCART_CART_TYPE_DIAGNOSTIC (plus a
hypercart_cart_type_diagnostic_enabled filter override) to find the root
cause of the cart/checkout fatal "TypeError: Unsupported operand types:
float / string" in WC_Discounts::sort_by_price()
(universal-child-theme issue #888).

Two error-level events:

- cart_type_corruption: snapshot cart item quantity/price/discounted_price
  at woocommerce_before_calculate_totals priority PHP_INT_MIN, re-check at
  PHP_INT_MAX. Reports each non-numeric value with type-safe rendering
  (objects/arrays never string-cast), origin attribution
  (upstream / hook_callback / added_during_hook), applied coupons, user id,
  request context, and callback lists for the six hooks able to write cart
  item values.
- cart_fatal_captured: register_shutdown_function catcher that matches the
  TypeError itself and dumps per-item quantity/price types from the
  in-memory cart. Covers paths that never fire the totals hook —
  WC_Cart::apply_coupon() validates via new WC_Discounts(WC()->cart)
  before any totals calculation.

Safety: all entry points swallow Throwable so the tracer can never take
down the cart it observes; re-entrancy guard on the snapshot; log volume
capped at 5 events per PHP process with signature de-duplication across
repeat hook firings.

In WC 10.8.x only a non-numeric string quantity can raise this fatal
(price is float-cast in WC_Discounts and WC_Cart_Totals), so quantity
findings are the authoritative signal.

Verified end to end on the Local site with a temporary corrupting
mu-plugin: hook-window capture attributed the culprit by file and line
({closure@zz-cart-diag-corruptor-TEMP.php:12}), and the shutdown catcher
captured a real resulting fatal (float * string in USPS weight-based
shipping) with a full cart type dump. 142 PHPUnit tests pass (19 new in
tests/CartDiagTest.php).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>