Completes incomplete certificate chains by fetching the missing intermediate
from the leaf's Authority Information Access URL, the way browsers do. A
server that sends its leaf without the intermediates above it now verifies
instead of failing the handshake.

Every fetched certificate must have genuinely signed the one that named it,
and the walk terminates only at a certificate signed by a root already in the
system trust store. A confirmation handshake discards the result if the host
still does not verify, so expired, revoked, self-signed and wrong-hostname
certificates are still rejected. Verification cannot be disabled.

Minor bump: cryptography>=42 is a new core dependency. It is imported lazily,
so a session that never meets a broken chain does not pay for it.

See docs/ref-tls-chain.md.