test(generate): lock in mkdirat path capture at args[1]

Audit of mkdirat(2) found the tracing implementation correct: the generated
BPF handler reads the pathname from args[1] (after the dirfd at args[0]),
while the sibling mkdir(2) reads from args[0] (no dirfd). Both are
KindPathname / FamilyFS with an UNCLASSIFIED return, consistent with
mknod/mknodat and docs/syscall-tracing-plan.md. The arg index is data-driven
from the kernel format via FieldNumber, so no source change was needed.

Add lock-in unit tests and real-format fixtures asserting:
- mkdirat captures the path from args[1], NOT args[0] (negative guard)
- mkdir captures the path from args[0]
- mkdirat/mkdir/mknodat share FamilyFS and KindPathname
- FieldNumber(pathname) = 1 for mkdirat, 0 for mkdir

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

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