net.quic: use crypto.rand instead of rand for connection IDs and TLS random

dial() generated original_dcid, scid, and the TLS 1.3 ClientHello.random
using the plain `rand` module, which defaults to WyRandRNG — a fast,
explicitly non-cryptographic PRNG. RFC 9000 §8.1 expects real entropy in
connection IDs, and ClientHello.random has its own TLS security
properties. Swap to crypto.rand, an OS-backed CSPRNG with an identical
bytes() signature already used elsewhere in the codebase (e.g.
crypto.ecdsa).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: Richard Wheeler

vlib/net/quic/conn.v

@@ -1,7 +1,7 @@
 module quic
 
 import time
-import rand
+import crypto.rand
 
 // RFC 9000/9001 — QuicConn is the top-level connection struct wiring
 // together every independently-built piece from Phases 0-8: packet/header