perf(cart): reuse precomputed subtotals in checkout summary

[wenhaog7]

Summary
This PR optimizes cart summary calculation and improves cart query performance in checkout flows.
Changes

• Reused precomputed item subtotals in xformCartCheckout when taxSummary is present, instead of always recalculating from unit price × quantity.
• Refactored fulfillment and surcharge aggregation loops for simpler and more efficient summary computation.
• Updated accountCartByAccountId resolver to decode opaque IDs once and reuse decoded values.
• Added a short-lived cache hint (maxAge: 60) for accountCartByAccountId to reduce load from frequent cart polling.
  Why
• Reduce repeated compute work on high-frequency cart reads.
• Keep summary calculations aligned with transformed item-level data.
• Improve responsiveness for storefront cart/checkout polling.
  Testing
• Manual checkout smoke test with multi-item carts.
• Verified cart query behavior with opaque and non-opaque IDs.
• Confirmed API and storefront cart flow remain operational.
  Notes
• No schema changes.
• No migrations required.

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Latest commit: 445fce3

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