Add pluggable FoX_error_handler hook

[jameskermode]

Summary

• Add an optional procedure-pointer hook in m_common_error (FoX_set_error_handler) that FoX_error_base consults before its historical stop.
• Lets a host library (e.g. libAtoms) install its own terminator so XML parse errors can propagate as language-level exceptions instead of silently killing the process.
• Historical stop path is preserved when no handler is registered.

Motivation

Needed to fix libAtoms/QUIP#724 — quippy's Potential(param_filename=...) silently exits the Python interpreter on malformed XML because FoX calls Fortran stop directly, bypassing libAtoms' system_abort → f90wrap_abort path. With this hook, libAtoms registers a handler that routes through system_abort, so Python sees a catchable RuntimeError.

Companion QUIP PR will register the handler in system_initialise.

Test plan

• Builds cleanly as part of QUIP's meson build.
• QUIP unit tests pass (51/51, including new tests/test_fox_xml_errors.py and the existing test_error_handling_raises_exceptions).
• Standalone Fortran callers unaffected — when no handler is registered, behaviour is identical (stop after stderr message).

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