Support linux-gnu host compilation for skip export; enable Linux CI matrix

[WiesnerPeti]

skip export runs a host-platform swift build in which the skipstone plugin performs its transpile/bridging analysis. On a Linux CI host that build compiles this package's non-Android branch, which currently assumes Darwin. This PR makes that linux-gnu host pass compile and activates the Linux element of the CI test matrix.

Source changes

• Logger: neither AndroidLogging nor OSLog exists on linux-gnu — add a compile-only shim in the final #else (never compiled on Android or Darwin).
• URL.applicationSupportDirectory: missing from Linux Foundation — extend the existing Android guard to os(Linux).
• AndroidBundle: subclasses Bundle, which Linux corelibs cannot inherit from — scope the file to the platforms that use it (Android/Robolectric/Darwin).
• AndroidLocalizedStringResource: scope the two AndroidBundle-typed convenience inits the same way.

CI change

Uncomment the runs-on line in .github/workflows/ci.yml to add ubuntu-24.04 to the matrix, matching skip-bridge and the other frameworks that build/test on Linux. Export stays enabled — that's what the source changes above make possible.

Safety

No behavior change on Android or Darwin: every source edit only affects what the linux-gnu host pass compiles. macOS swift build verified green.

[cla-bot]

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[marcprux]

For the Linux CI failure:

[219/239] Compiling SkipAndroidBridgeSamples SkipAndroidBridgeSamples.swift
/home/runner/work/skip-android-bridge/skip-android-bridge/Sources/SkipAndroidBridgeSamples/SkipAndroidBridgeSamples.swift:43:18: error: cannot find type 'LocalizedStringResource' in scope
41 | 
42 | public func localizedStringResourceLiteralKey() -> String {
43 |     let literal: LocalizedStringResource = "literal"
   |                  `- error: cannot find type 'LocalizedStringResource' in scope
44 |     return literal.key
45 | }

...you can probably just gate it with #if canImport(Darwin) since LocalizedStringResource is not available on non-Darwin platforms (see https://forums.swift.org/t/missing-localizedstringresource/80897/3).

[WiesnerPeti]

recheck

[marcprux]

Great contribution, thanks!