Add isolated RGB stock snapshots

[Jainakin]

Summary

• add an explicit Stock::snapshot() API backed by clone_no_persistence()
• guarantee that staged stock mutations cannot modify the source persistence providers
• cover source/snapshot isolation with a focused unit test

Motivation

Transactional RGB acceptance needs to validate and stage state changes without publishing partial stock state. This small primitive makes that ownership boundary explicit and testable.

Validation

• cargo +nightly fmt --all -- --check
• cargo test --workspace --all-features
• Result: 39 passed, 0 failed

Dependency

This PR is independent at the source level. It is consumed by the transactional acceptance work in rgb-lib.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[St333p]

Thanks for this PR!
However, a rather large change is underway to move RGB internal storage to a sqlite DB, and this PR might become unnecessary after that change. We expect to publish it in the next couple of weeks