Linux: call statx through syscall(2) so release builds link
The v0.63.0 Linux ARM64 and x64 release builds failed to link: undefined reference to 'statx' from bash_background/persistence.rs (file_mount_identity, the storage-cleanup mount check). The release builds run in cross 0.2.5 images whose glibc predates 2.28, the first glibc with a statx wrapper. Regular CI doesn't build that way, so the break first showed at the tag. Nothing was published.
The call now goes through syscall(SYS_statx, ...), which needs no glibc symbol. A kernel without statx returns ENOSYS, which refuses the repair like any other mount boundary AFT can't verify.
Verified: a release.yml validation run on this branch (run 40760225307, publishing off) built Linux ARM64 and Linux x64 successfully. Its CI-gate job fails by design, because branch runs have no main test run to point at.
Summary by cubic
Fixes the v0.63.0 Linux ARM64 and x64 release builds, which failed to link with undefined reference to 'statx'. The release builds link against a glibc older than 2.28, which has no statx wrapper, so the storage-cleanup mount check now calls statx through syscall(2) instead; a kernel without statx still refuses the repair with ENOSYS. Also extends the callgraph respawn test's child-barrier wait from 10 s to 60 s, since a loaded macOS runner could exceed the old deadline. Nothing was published from the broken build.
Also carries a test-only fix: issue_250_respawn_loop_converges_or_trips_without_false_readiness gave its child test process 10 s to reach a barrier. The test takes 12.5 s on main's macOS runners and failed at 10.7 s twice on this PR's macOS advisory runner. The wait is now 60 s.