Harden UUIDv7 custom RNG guidance

[dexcompiler]

Summary

• document the UuidV7Factory custom RNG contract in XML docs, README, the UUIDv7 guide, and changelog
• add C# tests showing same-seed deterministic replay and derived deterministic streams for independent test nodes
• add an F# property proving deterministic RNG replay for same seed/time/call pattern
• replace HashCode.Combine in deterministic RNG derivation with a stable explicit integer mixer for replayable derived streams
• make the UUIDv7 demo call out that deterministic RNG injection is for tests/simulations, not production UUID issuance

Why

UuidV7Factory accepts a caller-provided RandomNumberGenerator for deterministic testing and simulation replay. That capability is valuable, but identical deterministic RNG state plus identical time and call patterns intentionally reproduces UUID output. The production path should remain rng: null or an independent CSPRNG per factory.

Validation

• dotnet test tests/Clockworks.Tests.csproj --filter UuidV7FactoryTests -v minimal
• dotnet test property-tests/Clockworks.PropertyTests.fsproj --filter "Deterministic RNG replays UUID sequence" -v minimal
• dotnet test Clockworks.sln -v minimal -m:1
• DOCS_VERSION=dev npm run docs:build

Fixes #72

[dexcompiler]

@codex[agent] review this pr

[chatgpt-codex-connector]

To use Codex here, create a Codex account and connect to github.

[Codex]

@codex[agent] review this pr

Reviewed; added follow-up commit 16e1fa4 to (1) remove brittle claims about System.Random’s internal algorithm in deterministic RNG XML docs, (2) fix the stray // <summary> in the demo RNG, and (3) update the UUIDv7 guide snippet to avoid referencing a non-library DeterministicRandomNumberGenerator type. Targeted UUIDv7 replay tests still pass.