Compose host entry point: a @Composable that draws a Python-declared root and recomposes when it is replaced #18

pythonx-compose's declarative app root (thisisthepy/pythonx-compose#11): @app declares the root; the root lives in a Compose MutableState held by the Python runtime, so redeclaring it is an ordinary state write. The host needs a Kotlin @Composable that reads that state inside the composition and composes the Python root, so a replacement recomposes. Today PythonComposition exists only in ksp-fixtures/compose (test code).

Design (decided by the coordinator):

• A generic entry point, e.g. PythonContent(root: PyObject) / PythonContent(module = "…", attribute = "…"): the host passes where the root state lives; the binder knows no library names (no pythonx anywhere).
• The python-multiplatform library does not depend on Compose. The entry point goes in a new module python-multiplatform-compose (same naming as -ksp / -gradle-plugin), promoted from ksp-fixtures' PythonComposition/PythonComposed, which then use it.
• Desktop first, then Android (same jvm code); iOS/wasm later.

Completion: a compose fixture test where Python declares a root, the window renders it, Python replaces the root (state write) and the next frame shows the new content without any host-side call; replacing back works; the composition disposes Python callables on dispose. All fixture modules and desktopTest stay green.