Correct two documented claims that measurement contradicts

[sveneberth]

Two statements in AGENTS.md did not survive being measured. Both are documentation-only
changes.

Klarna does work server-side

AGENTS.md stated flatly that the client-side payment types "cannot be exercised
server-side", and that creating one "yields an empty resource that the provider rejects
further down the flow — Klarna answers COR.800.400.160".

Measured against the sandbox, following the flow to the end:

1. createPaymentType(Klarna()) — empty body, as the file describes
2. authorize with customerId, basketId and a public returnUrl → pending, plus a
   redirectUrl
3. the customer confirms on the Klarna page behind that URL (playground login: any German
   test number, any 6-digit OTP except 999999)
4. the payment then reports:

state    : pending
amount   : {"total": "100.0000", "charged": "0.0000", "remaining": "100.0000"}
  authorize    success    amount=100.0000

No COR.800.400.160. So the empty resource is filled in by the redirect step, and what
fails is skipping that step — not creating the type server-side.

The other affected types (Card, PayPal, Apple Pay, Google Pay, iDEAL, Click to Pay) were
not re-measured. The claim is therefore marked unverified for them rather than turned
around wholesale. The sandbox suite still does not drive Klarna, for the reason the file
now gives: the redirect step needs a browser and would place a real order on every run.

The item-rounding hazard is not theoretical

The note that BasketItem.serialize does not round while Basket.serialize does (#18)
read like a corner case about float residue. It is reachable with ordinary arithmetic: a
three-unit line with amountVat=15.97 * 3 produces 47.910000000000004, and the API
refuses the whole basket with

API.600.200.133  Amount 47.910000000000004 has more than 4 positions after the decimal point.

That error code appeared nowhere in the repository. Until #18 is fixed, a caller has to
round every item amount itself — worth knowing at the point where the rounding rule is
documented.

Verification

• pytest — 291 passed, 31 deselected
• pycodestyle --diff — clean
• No source changes; AGENTS.md and one test module docstring only.