Finish the price-math extraction: Checkout.tsx still hand-rolls 7 inline copies of usdcToFiat (FIN 51)

[gitchadd]

SEC	UX	AUTO	YAGNI	DELTA	FIN
0	5	90	90	70	51

Found by: adversarial review of #50 (architecture / YAGNI lens). Not a correctness bug — the inline copies are byte-identical floor-division, so they produce the same result — a maintainability + doc-accuracy issue and a good fix run candidate.

Symptom

The price-math.ts docstring claims usdcToFiat "mirrors the breakdown math the UI already does inline so the two stay in lockstep":

widgets/src/core/price-math.ts

Lines 9 to 11 in 8c08cd8

	* fiat-denominated checkout mode (`fiatChargeAmount`) is the only consumer of
	* the inversion; the forward `usdcToFiat` mirrors the breakdown math the UI
	* already does inline so the two stay in lockstep.

Nothing enforces lockstep. Checkout.tsx does not import price-math at all and still carries 7 inline (x * buyPrice) / 1_000_000n copies — lines 117, 145, 160, 164, 165, 224, 237:

widgets/src/widgets/Checkout.tsx

Lines 145 to 165 in 8c08cd8

	const chargedFiat = (chargedUsdc * state.buyPrice) / 1_000_000n;
	// Fee follows the DELTA (charged amount), not the gross order:
	// - credit covers fully (chargedUsdc == 0): no Diamond order, no fee
	// - chargedUsdc > 0 + chargedUsdc ≤ smallOrderThreshold: fee applies
	// - chargedUsdc > smallOrderThreshold: waived
	// Mirrors the Diamond's on-chain fee logic, which evaluates the
	// small-order threshold against `order.amount` (= delta when credit
	// is netted on-chain by the integrator).
	const feeUsdc =
	state.smallOrderThreshold !== null &&
	state.smallOrderFixedFee !== null &&
	chargedUsdc > 0n &&
	chargedUsdc <= state.smallOrderThreshold
	? state.smallOrderFixedFee
	: 0n;
	const feeFiat = (feeUsdc * state.buyPrice) / 1_000_000n;
	const totalFiat = chargedFiat + feeFiat;
	// Subtotal-without-credit: shown when credit > 0 to make the deduction
	// visible. Equals what the user would have paid pre-credit.
	const grossFiat = (orderUsdc * state.buyPrice) / 1_000_000n;
	const creditFiat = (credit * state.buyPrice) / 1_000_000n;

#50 added a fresh copy at L117 (feeFiatLabel) and edited L164 (grossFiat) rather than switching to the helper it was extracting in the same commit. order-machine.ts did adopt the helper (the good half); the view did not — so there are now two sources of truth for one formula that agree only by coincidence.

Fix (mechanical, behavior-preserving)

• Import usdcToFiat into Checkout.tsx; replace the 7 inline copies with usdcToFiat(a, state.buyPrice).
• Collapse the two near-identical gross-fiat IIFEs in order-machine.ts (routingFiat ~L728-737, contextFiat ~L768-785) into one shared price-math helper: grossFiat = usdcToFiat(order) + usdcToFiat(fee).

Net effect: every "amount → fiat" number in the buy path derives from price-math, making the docstring's lockstep claim structurally true. Deletes duplication, adds zero surface.

Doc nit (bundle)

README says the "You pay" line "lands on exactly the number you passed":

widgets/README.md

Lines 1266 to 1270 in 8c08cd8

	the protocol's small-order fee is **baked in**, not added on top. The
	widget reads `getPriceConfig(currency).buyPrice`, backs the fee out of the
	total, and derives the USDC order amount so the **"You pay"** line lands on
	exactly the number you passed. (Compare `usdcAmount`, where the fee is
	charged *on top* of the USDC you specify.)

It lands to the nearest micro-USDC (round-half-up + floor re-display leaves up to a ~½-micro-USDC-of-price residual; the round-trip test asserts diff <= BUY_PRICE, i.e. explicitly not exact). Soften "exactly" to "to the nearest micro-USDC".