fix(cmake): respect explicit PYBIND11_USE_CROSSCOMPILING under CMP0190 (#6094)

* fix(cmake): respect explicit PYBIND11_USE_CROSSCOMPILING under CMP0190

PR #5829 added `AND NOT DEFINED CMAKE_CROSSCOMPILING_EMULATOR` to the
condition setting `_PYBIND11_CROSSCOMPILING`. That gate was meant to only
control the auto-default of `PYBIND11_USE_CROSSCOMPILING`, but it ended up
overriding a value the project set explicitly. Emscripten/Pyodide defines
`CMAKE_CROSSCOMPILING_EMULATOR` (node) yet still wants cross-compiling
mode, so the flag was forced OFF and the build failed with a spurious
"Cannot run the interpreter" / pointer-size mismatch.

Move the emulator check up so it only guards the auto-default, and never
override an explicitly set `PYBIND11_USE_CROSSCOMPILING`.

Fixes #6043

Assisted-by: ClaudeCode:claude-opus-4.8

* fix(cmake): keep CMP0190 auto-default in add_subdirectory builds

The previous commit's `NOT DEFINED PYBIND11_USE_CROSSCOMPILING` guard
broke the intended auto-default for top-level / add_subdirectory builds:
CMakeLists.txt runs `option(PYBIND11_USE_CROSSCOMPILING ... OFF)` before
including pybind11Common.cmake, so the variable is already defined (OFF)
and the CMP0190 default never applied.

Capture, before the option() call, whether the project set the value
itself, and let pybind11Common.cmake apply its default unless the value
was set explicitly. This keeps both the emulator override fix and the
CMP0190 auto-default for subdirectory consumers.

Assisted-by: ClaudeCode:claude-opus-4.8