Build: sample iOS via the plugin; remove the library's duplicate staging (#321) - #327

Closes #321 (follow-up of #90).

• sample:
  • Removed its prepareIosFrameworks Sync and the hand-added -framework Python linker options, which the plugin's preparePythonXcframework now provides. -Objc stays.
  • The Xcode phase runs :sample:installPythonXcodeScript and then the installed build/python-multiplatform/xcode/install-python.sh. PYTHON_PAYLOAD_DIR is unchanged.
  • Only iOS wiring changed: sample already applied the bindings plugin.
• python-multiplatform:
  • Removed stageIosPythonHome_, stageIosPythonHome and stageIosPythonHomeForXcode. Their only user was sample's old phase (grep covered .github/workflows; tools/ does not exist), and the library's task list no longer has them.
  • Kept: downloadPython_ios with its checksum and sigstore (the library's cinterop and iOS tests), extractIosSimulatorStdlib and SIMCTL_CHILD wiring (iOS tests), and :python-multiplatform:embedAndSignAppleFrameworkForXcode (sample's compile phase).
• install-python.sh: the stale default (the removed library task) is now a placeholder that installPythonXcodeScript bakes in. A raw run without PYTHON_HOME_TASK stops with a message. I tested both the raw and the baked copy.
• Docs: ios-app-bundle.md, SPEC, ROADMAP and the iosMain README now describe the plugin path.
• Evidence:
  • Plugin tests 350/350.
  • :sample:preparePythonXcframework, linkDebugFrameworkIosSimulatorArm64, installPythonXcodeScript and stageIosPythonHomeForXcode pass, and the staging prints PYTHON_HOME_DIR= / PYTHON_DYLIB_INFO_TEMPLATE= under sample/build.
  • xcodebuild of the sample simulator app: BUILD SUCCEEDED. On the simulator: 'DEMO 1 runtime | 3.15.0 · sys.platform=ios' and 'DEMO 8 bundle' with the payload greeting, _json via .fwork and the stdlib under PythonDemo.app/python-multiplatform-home.
  • The library's iOS simulator and androidNative compiles pass.
  • Note: Xcode's Gradle phase needs JAVA_HOME at JDK 21 exported (the default JDK 25 breaks Gradle 8.11).