test(chmod): add end-to-end coverage for chmod/fchmod/fchmodat family

chmod/fchmod/fchmodat/fchmodat2 previously had no integration coverage:
no scenario in cmd/ioworkload/ and no test in integrationtests/. All four
are correctly classified (chmod KindPathname args[0]; fchmodat/fchmodat2
KindPathname args[1]; fchmod KindFd args[0]; all FamilyFS, all UNCLASSIFIED
ret) but nothing exercised them end-to-end.

Add a chmod-basic scenario that, on a temp file the caller owns (so all
calls are unprivileged), issues raw chmod(path, 0640), fchmodat(AT_FDCWD,
path, 0644, 0), fchmod(fd, 0644), and best-effort fchmodat2(AT_FDCWD, path,
0640, 0) (raw syscall 452, ENOSYS tolerated on kernels < 6.6). Raw syscalls
are used so each distinct tracepoint fires rather than glibc redirecting
chmod to fchmodat.

TestChmodBasic asserts enter_chmod and enter_fchmodat capture the file path
(chmodfile.txt) and enter_fchmod fires (KindFd, no path). fchmodat2 is not
asserted since it is version-gated, though it does fire on current kernels.

Mirrors the utime coverage (scenario_utime.go / utime_test.go).

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

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