Clarify UUIDv7 collision and skew guarantees

[dexcompiler]

Summary

• clarify UuidV7Factory and IUuidV7Factory XML docs so per-instance monotonic allocation is not confused with deterministic global uniqueness
• add a dedicated UUIDv7 guide section for collision and clock-skew semantics across one factory, multiple threads, multiple factories, fleets, and restarts
• update README and changelog docs with singleton guidance, probabilistic cross-factory uniqueness, and storage uniqueness constraints for high-assurance shared namespaces

Why

UuidV7Factory already handles backward wall-clock movement inside one live factory instance by continuing from its CAS-protected logical (timestamp, counter) frontier. The previous docs did not draw a sharp enough line between that deterministic local guarantee and probabilistic cross-process/fleet uniqueness.

Validation

• dotnet test Clockworks.sln -v minimal -m:1
• cp assets/clockworks-display-resized.png docs/public/logo.png
• DOCS_VERSION=dev npm run docs:build

Note: npm ci in docs/ reported existing moderate dependency advisories but completed successfully; this PR does not modify docs dependencies.

Fixes #67