Plugin: consumer iOS wiring (Python.xcframework + stdlib staging) (#90) - #320

Closes #90 (part 1: plugin and consumer fixture. Switching sample/ and removing the library-side duplicates is a follow-up PR, as the leader allowed.)

Approved design, DSL (A):

• On a module with Kotlin/Native iOS targets only, the plugin registers:
  • downloadPython_ios: the pinned iOS support archive, written only to the consumer's build/python-standalone/. Its SHA-256 is baked into the plugin from python-checksums.properties (ios-3.15.0-pythonorg). A mismatch fails the build: a bad cached file is deleted and re-downloaded once, and the second copy must verify. There is no sigstore check in the consumer path; the pinned hash is the trust anchor.
  • preparePythonXcframework: a Sync into build/xcode-frameworks. It adds -framework Python -F to every iOS framework binary and makes the link tasks depend on it.
  • stageIosPythonHome_, stageIosPythonHome, stageIosPythonHomeForXcode: the same names and stdout contract as the library's.
  • installPythonXcodeScript: copies install-python.sh with :stageIosPythonHomeForXcode as its default task.
• pythonBindings { ios { linkPythonFramework.set(false) } } turns off the linker options and the link dependency. I added the ios(Action) block function after the fixture showed that the block did not compile without it. IosConsumerWiringTest covers it.

Evidence (python-multiplatform-gradle-plugin/fixtures/ios-consumer, plugin only):

• Plugin tests: 340/340. Artifact fixture desktopTest and the library's iOS compile pass.
• linkDebugFrameworkIosSimulatorArm64 passes. With linkPythonFramework.set(false) the link fails with undefined _Py* symbols (mutant). The archive and xcframework land under the fixture's build/.
• xcodebuild of the simulator app succeeds. The bundle contains python-multiplatform-home/lib and python/probe_payload.py.
• On the simulator: IOS_CONSUMER_PROBE stdlib=ok ext=ok payload=ok (json, _json via .fwork, probe_payload).

Found on the way: the KSP processor emits an unresolved reference for a root-package function. That is #319; the fixture uses a package.