docs: warn about isPersisted vs isPersisted.promise footgun

[Danny-Devs]

Adds an IMPORTANT callout to the Transaction States section warning that await tx.isPersisted silently resolves immediately because Deferred is not a thenable. Users must use tx.isPersisted.promise.

This is a real-world bug we hit in production. TypeScript does not flag it because await accepts any. The consequence is that forms clear, navigation fires, and success messages display before the server confirms the write — and if the server rejects, the rollback happens silently with no error handling.

Context:

• Deferred<T> (defined in src/deferred.ts) has no .then() method
• await nonThenable resolves to the value itself per the JS spec
• Every doc example already uses .promise correctly, but there's no explicit warning about omitting it

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