Gradle plugin: consumer-side iOS wiring (Python.xcframework + stdlib staging) for apps outside this repo #90

#59 proved the iOS app bundle inside this repo, but its pieces live in python-multiplatform's own build scripts, so a consumer app (toolchain usage-example, real users) cannot get them from the published plugin/artifacts:

1. Python.xcframework: sample/build.gradle.kts copies it to sample/build/xcode-frameworks, adds linkerOpts("-framework","Python","-F…") to the KMP framework, and the Xcode project links/embeds it.
2. stdlib staging: stageIosPythonHomeForXcode exists only on :python-multiplatform; tools/xcode/install-python.sh runs it in the same Gradle root.

Scope: the published Gradle plugin (io.github.thisisthepy.python.multiplatform.bindings) provides, on the consumer project: (i) a task that stages Python.xcframework into the consumer's build/ and wires the linkerOpts into its Kotlin/Native iOS framework binaries automatically; (ii) stageIosPythonHomeForXcode (acquiring the pinned iOS support archive like the desktop staging does); install-python.sh usable from the consumer's Gradle root (and shipped/located in a way a consumer can call). sample/ switches to the plugin path where it only duplicates it.

Completion: a consumer fixture outside :python-multiplatform (e.g. a ksp-fixtures-style module or toolchain's usage-example via mavenLocal) builds an iOS simulator app using only the published plugin + artifacts, and the DEMO-8-style probe shows stdlib + payload imported from the bundle.