`a[0]?.b.c` read `.c` from undefined when the element was missing: the
tail only joined the guard when the checker typed the receiver nullable,
and an array or index-signature read is typed present while it lowers to
an undefined-armed value. The tail check now also accepts receivers that
can lower to a hidden undefined; present receivers re-dispatch the plain
lowering. A non-null assertion stays inside the chain (`a?.b!.c`), and
parentheses end it for checked-dynamic reads (`(a?.b).c` throws).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
