test(chown): add end-to-end coverage for chown/fchown/lchown/fchownat family

chown/lchown/fchownat/fchown previously had no integration coverage: no
scenario in cmd/ioworkload/ and no test in integrationtests/. All four are
correctly classified (chown/lchown KindPathname args[0]; fchownat KindPathname
args[1]; fchown KindFd args[0]; all FamilyFS, all UNCLASSIFIED ret) but nothing
exercised them end-to-end.

Add a chown-basic scenario that, on a temp file and a symlink the caller owns,
issues raw chown(path,-1,-1), lchown(symlink,-1,-1), fchownat(AT_FDCWD,path,
-1,-1,0) and fchown(fd,-1,-1). Owner/group -1/-1 means "leave both ids
unchanged", which the kernel accepts without CAP_CHOWN, so the scenario is
fully UNPRIVILEGED and nothing is actually modified. Raw syscalls are used so
each distinct tracepoint fires rather than glibc redirecting chown to fchownat.

TestChownBasic asserts enter_chown and enter_fchownat capture the regular file
path (chownfile.txt), enter_lchown captures the symlink (chownlink), and
enter_fchown fires (KindFd, no path). Mirrors the chmod coverage
(scenario_chmod.go / chmod_test.go).

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

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