ScrollViewWithOnScrollChanged: move onto onScrollGeometryChange or deprecate it #25

Summary

ScrollViewWithOnScrollChanged (Components/Geometry/ScrollViewWithOnScrollChanged.swift) reports the scroll offset with a hand-rolled probe. The probe is a zero-size LocationReader placed at the top of the content, inside a named coordinate space. It exists because onScrollGeometryChange(for:of:action:) was not available. That API is iOS 18, and after #15 it is available at the package floor.

The system API reads contentOffset directly. The probe has two weaknesses the system API doesn't:

• it depends on the probe staying the first child of the content;
• it reports through onChange on every layout pass.

Why it was not done in #15

ScrollViewWithOnScrollChanged is public API. Removing or reworking it is a breaking change, and #15 deliberately left the public surface untouched.

Consumer

dashwallet-ios develop has one call site: DashWallet/Sources/UI/Menu/Settings/LocalCurrency/LocalCurrencyView.swift:146. That code uses onScrollChanged(offset) for its header.

Options

1. Keep the type and its signature, and reimplement it on onScrollGeometryChange. This is not a breaking change. The sign and origin of the offset must stay the same as today's LocationReader value.
2. Deprecate it (@available(*, deprecated, message: "Use onScrollGeometryChange")), move LocalCurrencyView to the system API, and remove the type in a later release.

Whichever option is chosen, update docs/utilities.md, docs/README.md and the README table.