Re-export every manifest module by one rule over the binder's Kotlin-named surface #8

What

One mechanism that makes each pythonx.compose.* module in pythonx-map.toml a Pythonic view of its androidx.compose.* module, plus a test harness that installs the binder the way an embedder does now (no register_package).

Why

The binder exposes Kotlin names only and publishes a contract for exactly this (PythonMultiplatform d00f413f, KotlinSurface.kt): inspect.signature(fn) with Kotlin parameter names and KOTLIN_DEFAULT, and python_multiplatform.describe(fn). AGENTS.md §12–13: rename here, by rule, no per-widget wrapper. The 37 failing tests call the removed register_package.

Proposed API (to be refined in the PR)

• pythonx/compose/_reexport.py holds the rule; each mapped package's __init__.py is one line that hands its module and Kotlin package to it, and gets a module __getattr__ / __dir__.
• Names: a type or @Composable (PascalCase) keeps its Kotlin name; a camelCase function or property is reached by its snake_case name (fill_max_width → fillMaxWidth). The inverse is looked up in the module's real names, never computed, so toURLString stays reachable.
• Keywords: on_click= → onClick=, mapped against the declaration's own parameter names from describe() (all overloads), so a keyword that matches no parameter is an error naming the valid ones.
• Signatures: each re-exported callable answers inspect.signature with the snake_case names; defaults stay KOTLIN_DEFAULT.
• Overloads: passed through; the binder dispatches.
• Receiver methods: extension methods on returned proxies (Modifier.padding(16).fill_max_width()) resolve by the same rule.
• Value classes: the manifest's raw-primitive-allowed is applied through python_multiplatform.binding.allow_raw_primitive when the package is imported.

Done when

• tests/adapter.py no longer calls register_package; it installs python_multiplatform + python_multiplatform.binding from the binder source and imports pythonx.compose.* from disk.
• The 37 tests pass with a PythonMultiplatform checkout (rewritten to the snake_case surface where they used the old one), and still skip without one.
• A test fails when the rule is disabled (AGENTS.md §8).