Thread safety assertion woes

We had a pull request to relax the thread safety checks in https://github.com/tursodatabase/libsql/pull/1887. However we ended up opting for for https://github.com/tursodatabase/libsql/pull/1891, but it requires clients to rely on something we tag as unsafe. Let's look into fixing thread safety checks in a safe way.

The issue manifests, for example, with some Tauri apps:

Failed to initialize embedded replica: ❌ spawn_blocking task failed: task 48 panicked with message "assertion `left == right` failed: libsql was configured with an incorrect threading configuration and\n                    the api is not safe to use. Please check that no multi-thread options have\n                    been set. If nothing was configured then please open an issue at:\n                    https://github.com/libsql/libsql\n  left: 21\n right: 0"

Full Discord thread here: https://discord.com/channels/933071162680958986/1379937960547586098

Start by reviewing pull requests 1887 and 1891 to understand the threading-check alternatives, then read the linked Discord discussion for the Tauri failure context. Reproduce or inspect the embedded replica initialization path and define a safe resolution for the assertion without requiring clients to rely on an unsafe configuration.