chore: lint the -inl.h invariants of the precompiled mode

tools/check_inl_headers.py checks that each namespace-scope definition
in an -inl.h file has PYBIND11_INLINE, and that each macro in a
preprocessor condition is known: either the same for the library and
the module, or encoded in the precompiled config guard. Also document
that a custom PYBIND11_NAMESPACE must reach the library.

Assisted-by: ClaudeCode:claude-opus-5-5