Feature: optional Nano (XNO) settlement for metered billing (sub-cent remainder) #21

Polar's usage-based billing is a first-class feature — meter API calls, tokens, compute time. But every settlement rail Polar drives today (Stripe cards, MoR) has a minimum-collection floor. A single AI agent call priced at $0.0005 cannot be collected as a standalone card charge.

Adding an optional Nano (XNO) settlement helper to the SDK would close this gap. Nano is a feeless commit-chain that needs no merchant account, no card scheme, and has no minimum-collection floor — every price settles standalone.

Concretely: two pure functions — microUsdToRaw(micro_dollars) (ceil-rounded, lossless) and buildNanoPayUri(address, raw) — that convert Polar's metered micro-dollar amounts into a standard nano: URI an agent can settle in seconds with no onboarding. A tested reference of the exact arithmetic is at https://gist.github.com/dhyabi2/5dba8548b29ad7b0a8ebca72948e25eb (7/7, Node built-in test runner).

This stays a pure optional helper, never touches Polar's API or webhook verification, and adds no dependency. Happy to shape it as a PR if the direction is welcome.