test(xattr): add path-based setxattr end-to-end integration coverage

The xattr integration tests previously only asserted on getxattrat/
listxattrat (path/dirfd READ-classified variants). The path-based
setxattr(2) was traced by the existing xattr-getxattrat scenario
(workload calls syscall.Setxattr; xattrTraceArgs includes setxattr) but
never asserted. Add TestXattrSetxattr to verify end-to-end that:

  - enter_setxattr captures the filesystem PATH at args[0] (kind=pathname),
    never the xattr NAME at args[1];
  - exit_setxattr is UNCLASSIFIED: setxattr returns a 0/-1 status, not a
    byte count (the size arg is the INPUT value length), so accounted
    bytes must be exactly zero. This guards against the msgsnd-style bug
    of treating a status return as bytes written, and contrasts with
    getxattr/listxattr which DO return byte counts (READ-classified).

This is the PATH-based set complement to filed task 8i0 (fd-based
fsetxattr/fgetxattr/flistxattr/fremovexattr); it does not duplicate it.
Classification (FamilyFS, KindPathname enter, UNCLASSIFIED ret) is
verified by inspection per task guidance, not by a unit test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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