crypto/x509: normalize IP name constraints with non-zero host bits

Name constraint IP subtrees are encoded as a network address followed by
a mask. RFC 5280 permits the network address to carry non-zero host bits
(e.g. 10.10.10.10/16, denoting 10.10.0.0/16), and some CA tooling emits
this verbatim. parseNameConstraintsExtension stores the address exactly
as encoded.

The sub-quadratic constraint matcher added in Go 1.26 (constraints.go)
binary-searches the sorted constraint set using the raw stored IP and
then checks only the nearest lower-bound entry with net.IPNet.Contains.
That relies on the stored IP being the canonical masked network address.
When the address has host bits set, the containing network can sort above
the target while a lower-addressed constraint occupies the neighbor slot,
so the containing network is never tested and search reports no match.

Because this only ever yields false negatives, an excluded constraint
with host bits set could fail to match an address inside its network,
silently accepting a certificate the constraint should have rejected.

Normalize the host bits in newIPNetConstraints, where the matcher builds
its sorted set, instead of in the parser. This restores the matcher's
invariant while leaving the parsed PermittedIPRanges and ExcludedIPRanges
with the address exactly as encoded in the certificate.

Fixes #79833

Change-Id: I7ec6b0452871d32a2f3db2eb9d46802ce64ecd78
Reviewed-on: https://go-review.googlesource.com/c/go/+/787220
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>