sendfile64: capture out_fd instead of dropping both fds

sendfile64(out_fd, in_fd, offset, count) transfers bytes between two file
descriptors in the kernel and returns the number of bytes written to out_fd.
Its tracepoint fields carry no field literally named "fd", so it fell through
to KindNull and captured no descriptor at all - inconsistent with its sibling
copy_file_range (KindFd) and the read/write/sendto/recvfrom families.

Add an explicit sys_enter_sendfile64 -> KindFd override that captures out_fd
(args[0], the destination the bytes are written to), matching the single-fd
KindFd convention. The return value stays TransferClassified, consistent with
copy_file_range/splice/tee/vmsplice. Family stays Network (sendfile is
historically socket-oriented; copy_file_range=FS is pure file-to-file).

Update docs/syscall-tracing-plan.md (move sendfile64 from null to fd kind),
regenerate C/Go artifacts, fix the phase-A classify assertion, and add
TestClassifySendfile64CapturesOutFd as a lock-in + negative test. The existing
TestRetbytesPhaseA integration test still passes with the runtime change.

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

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