test(mknod): lock in mknod/mknodat path-arg classification

Audit of mknod(2) found the tracing implementation already correct:
sys_enter_mknod captures the real pathname from args[0] (no dirfd),
while the sibling sys_enter_mknodat captures it from args[1] (after
dirfd). Both are FamilyFS path_events; both exits are ret_event
UNCLASSIFIED (int 0/-1). No code or doc changes were needed.

Add lock-in tests guarding this behavior against regressions:
- TestGenerateMknodMknodatHandlers asserts the generated BPF C reads
  the path from args[0] for mknod and args[1] for mknodat.
- FormatMknodat/FormatExitMknodat testdata mirroring the real
  tracepoint layout (dfd pushes filename to args[1]).
- mknodat rows added to the classify kind (KindPathname) and family
  (FamilyFS) test tables, matching the existing mknod coverage.

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

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