test(xattr): cover get/list/lset/setxattrat/removexattr + fd-based xattr variants

Extend the xattr scenario + integration test to exercise end-to-end the
xattr syscall variants that previously had no coverage (only setxattr and
the -at get/list/remove variants were tested). Classifications were already
verified correct by inspection; this is pure coverage hardening.

READ-classified variants (assert enter path/fd + bytes>=1):
- getxattr(path,name,...)   READ, KindPathname@arg0   (ej0)
- lgetxattr(path,name,...)  READ, KindPathname@arg0   (oj0)
- listxattr(path,...)       READ, KindPathname@arg0   (rj0)
- llistxattr(path,...)      READ, KindPathname@arg0   (rj0)
- fgetxattr(fd,...)         READ, KindFd@arg0         (8i0)
- flistxattr(fd,...)        READ, KindFd@arg0         (8i0)

UNCLASSIFIED variants (assert enter path/fd + bytes==0):
- lsetxattr(path,...)       KindPathname@arg0         (cl0)
- setxattrat(dirfd,path,...) KindPathname@arg1        (vj0)
- removexattr(path,name)    KindPathname@arg0         (kj0)
- lremovexattr(path,name)   KindPathname@arg0         (kj0)
- fsetxattr(fd,...)         KindFd@arg0               (8i0)
- fremovexattr(fd,...)      KindFd@arg0               (8i0)

The l* GET/LIST/SET/REMOVE variants target a regular file (not a bare
symlink) so they fire deterministically: user.* xattrs on a symlink itself
are kernel-restricted (EPERM). setxattrat uses the raw syscall (nr 463,
Linux 6.13+) since Go does not export it; this kernel (7.0.9) supports it.

New scenarios use golang.org/x/sys/unix (raw syscalls, no glibc redirect)
so the exact tracepoints fire. New tests scope -trace-syscalls to exactly
the variant under test to avoid substring-match cross-contamination.

All 13 TestXattr* integration tests PASS under root (mage testWithName).

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

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