fix: restore macOS transparency and defer CSS errors (3.1.1)

[NuPlay]

Patch release. Fixes two regressions shipped in 3.1.0.

🐛 macOS backgrounds were no longer transparent

3.1.0 guarded the drawsBackground KVC call with responds(to:):

if webview.responds(to: NSSelectorFromString("setDrawsBackground:")) {
    webview.setValue(false, forKey: "drawsBackground")
} else {
    conf.errorHandler?(.webViewConfigurationFailed)
}

The premise was wrong. setValue(_:forKey:) does not need a setDrawsBackground: method — KVC falls back to _setDrawsBackground: and then to the _drawsBackground ivar. So the guard always took the else branch:

• the background was never made transparent
• every macOS launch emitted a spurious webViewConfigurationFailed

A guard intended to prevent a crash disabled the feature it was guarding.

Replaced with underPageBackgroundColor, which is public WebKit API available on iOS 15 and macOS 12 — exactly this package's deployment targets. No KVC, no private selectors.

🐛 cssGenerationFailed fired during a SwiftUI view update

The handler was called synchronously from loadHTMLIfNeeded, which runs inside makeUIView and updateUIView. A handler that mutates state there produces "Modifying state during view update, this will cause undefined behavior" — the exact warning #72 reported and #74 fixed. 3.1.0 reintroduced it.

Errors are now delivered on the next main-queue turn via reportError(_:).

🧪 Why 3.1.0 shipped without catching either

The 3.1.0 tests compared generated strings. Neither defect is visible that way.

The new tests check behavior instead:

• macOS transparency is asserted against a real WKWebView found inside a hosted NSHostingView
• callback timing is asserted by capturing whether the reporting call had already returned when the handler ran

📖 Documentation

Examples that did not compile:

• .blue / .cyan were ambiguous between the Color and NSColor/UIColor overloads → qualified as Color.blue
• let css was declared twice in one scope
• fullScreenCover(item:) was bound to a String?, which is not Identifiable, and is iOS-only → replaced with an Identifiable wrapper and .sheet

Also removed 18 trailing-whitespace errors, and normalized 17 British spellings (colour, behaviour) that disagreed with the API on the same line:

"Invalid hex colour in fontColor or linkColor"

📦 Release prep

• CHANGELOG entry for 3.1.1
• documented version floor raised to 3.1.1, since 3.1.0 is opaque on macOS

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