test(dup): lock in fd_event handler captures oldfd (args[0])

Audit of dup(2) found the tracing implementation already correct and
consistent with its dup2/dup3 siblings: dup(int oldfd) takes a single fd
argument (the sys_enter_dup tracepoint exposes it as field "fildes",
unsigned int, at args[0]). It is classified KindFd (a plain fd_event),
the enter handler captures ev->fd from args[0] per the KindFd
convention, it is in the FS family (fd grouping), and its exit returns
the new (lowest-numbered unused) descriptor or -1 as a plain UNCLASSIFIED
ret_event (never a byte-count transfer). Like dup2, dup carries no flags
and clears FD_CLOEXEC on the duplicate; the eventloop registerDup path
registers the returned newfd onto the same underlying file with flags=0,
which it already honors (applyFdTransferOp handles SYS_ENTER_DUP).

Docs (FS, fd) and the drift tests are in sync; existing coverage already
includes TestClassifyDup, the makeFdDupTestData full-lifecycle eventloop
test, and integration TestDupBasic/TestDupInvalidFd.

No discrepancies were found, so add a lock-in test (matching the dup2
audit) asserting the generated BPF C for dup captures fd from args[0]
(not args[1]), emits an fd_event (not a dup3_event), wires no flags, and
classifies the exit UNCLASSIFIED. Adds FormatExitDup testdata to drive
the exit handler assertions.

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

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