libyams_mobile stopped compiling without anyone noticing because no CI
lane built it: every lane forced YAMS_ENABLE_MOBILE_BINDINGS=false, and
the "Check mobile ABI surface" step was commented out and pointed at
scripts/ci/check_mobile_abi.sh, which was never committed.
Add a linux-x86_64-mobile matrix entry that configures with
enable-mobile-bindings and enable-mobile-tests, compiles only the
yams_mobile and catch2_mobile_abi targets, runs the ABI check, and runs
the mobile meson suite. Other lanes keep mobile bindings off.
Add scripts/ci/check_mobile_abi.sh. It compiles the public header as C11
and C++20 and takes the address of every YAMS_MOBILE_API function, so a
declaration hidden from C (the extern "C" bug fixed in the previous
commit) fails. It then checks that the library exports exactly the
declared yams_mobile_* functions with unmangled names.
This lane proves the Linux x86_64 host build only. It does not show that
an Android .so or iOS XCFramework loads on a device.
Signed-off-by: trvon <me@trevon.dev>
