Replace DefaultHasher + SystemTime + process::id() with UUID v4
(backed by getrandom/urandom CSPRNG) for pairing code generation.
The previous implementation used predictable entropy sources
(system time to ~1s precision and process ID) with a non-cryptographic
hash (SipHash), making the 6-digit code brute-forceable.
The new implementation extracts 4 random bytes from a UUID v4
(which uses the OS CSPRNG) and derives the 6-digit code from those.
No new dependencies added — reuses existing uuid crate.
Adds a test verifying non-deterministic output.
Ref: CWE-330 (Use of Insufficiently Random Values)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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