test: stabilize offline library snapshot coverage

[jdluu]

Summary

• Makes the snapshot retention test wait for the initial state before triggering a failed refresh.
• Verifies the prior snapshot remains available and the failure notification is emitted.
• Prevents asynchronous mock sequencing from making the suite nondeterministic.

Scope

Closes #111. Test-only maintenance within the active feature freeze. No production behavior or dependencies changed.

Validation

• Focused Vitest: 6 passed
• Full Vitest: 304 passed
• TypeScript build: passed
• Biome check: passed
• Rust tests: 226 passed
• git diff --check: passed