test(xattr): add removexattrat end-to-end integration coverage

removexattrat(2) (Linux 6.13+) was the one xattr *at-variant lacking
integration coverage: xattr_test.go exercised getxattrat/listxattrat
(READ-classified byte counts) and the path-based setxattr, but never the
REMOVE *at variant. Unlike its getxattrat/listxattrat siblings,
removexattrat returns a 0/-1 status (not a byte count), so its exit must
be UNCLASSIFIED — matching removexattr/lremovexattr/fremovexattr.

Add an ioworkload scenario (xattr-removexattrat) that setxattr's a
user.* attribute via a real path then removes it via raw
removexattrat(AT_FDCWD, path, 0, name), plus TestXattrRemovexattrat
asserting the path (args[1], after the dirfd) is captured (never the
xattr name at args[3]) and that accounted bytes are exactly zero
(guarding against wrongly READ-classifying it like getxattrat/
listxattrat). Distinct from the fd-based fremovexattr gap (task 8i0).

Classification verified by inspection: FamilyFS (xattr marker),
KindPathname at the pathname field (ctx->args[1]), and absent from the
ret-classification table => UNCLASSIFIED. No generator change needed.

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

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