[b-re-w]

Closes #161 (option B, user decision; replaces option A's refusal from #303).

• Public API change: withGIL(crossinline block) and Python3.withPython(crossinline block). A non-local return from those blocks no longer compiles.
• Breakage list:
  • This repository: no call site broke. Every target compiles: desktop main and test, androidNative, iOS main and test, wasmJs, Android, compose, the ksp/compose/artifact fixtures and sample. The only other change is the private helper PyObject.callWithTuple, whose fill block also became crossinline.
  • Other repositories: no Kotlin use of withGIL or withPython in pythonx-compose, pythonx-concurrent, pythonx-graphics, pythonx-platform, toolchain or torchnative (grep).
• Pin: on a JVM virtual thread, desktop only, the outermost scope runs as the comparator of C's bsearch over one element. That is an FFM upcall inside a downcall, so the native frame pins the virtual thread from PyGILState_Ensure to Release. An upcall must not throw, so the result or exception is carried out and rethrown. Platform threads and other targets run the block directly.
• Panama gains createUpcallStubII_I, (long, long) -> int, in both backends, with an @UpcallShape marker; the generated reachability metadata includes it. The bsearch downcall (5 x long -> long) is already a registered descriptor.
• SPEC T-1a is rewritten.
• Tests: VirtualThreadGILTest.
  • 16 virtual threads park 5 times each inside withGIL, and pthread_self must stay the same.
  • An exception passes through the pinned scope and the GIL is released.
  • Nesting works, and platform threads are unchanged.
  • Mutant: without the pin, the test JVM aborts (exit 134) in PyGILState_Release on the wrong OS thread, which is the corruption Binder: withGIL on a JVM virtual thread can release the GIL from another carrier #161 described.
  • ReachabilityMetadataTest is updated for the new shape. python-multiplatform desktopTest 644/644; ksp app 80, compose 129, artifact 50.
• Not verified: running the pin inside a GraalVM native image (rule 13). Only the descriptors are registered; no native-image build was run for this path.
• pythonx-concurrent: notified on Binder requests from pythonx-concurrent (tracking) #164.