Add f90wrap_stub.F90 to libAtoms build for standalone compilation

Added f90wrap_stub.F90 to the libAtoms meson build sources. This provides
a stub implementation of f90wrap_abort() that allows QUIP libraries to be
built standalone without f90wrap.

When building quippy with f90wrap, the real implementation will override
this stub. This resolves the circular dependency where:
- QUIP libraries need f90wrap_abort_() symbol to link
- f90wrap_abort_() is only provided during quippy build

With this change, QUIP can be built standalone, and then quippy can be
built on top with the proper f90wrap integration.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jameskermode

src/libAtoms/meson.build

@@ -1,4 +1,5 @@
 libAtoms_F90_sources = [
+  'f90wrap_stub.F90',  # Stub implementation for standalone builds
   'error.F90',
   'kind_module.F90',
   'System.F90',