feat(storage): Add instance-bound SafeBoxStateListener support to SafeBoxBlobStore

[harrytmthy]

Overview

This PR introduces support for instance-scoped SafeBoxStateListener, enabling consumers to observe the internal state of a specific SafeBox instance without relying on global observers.

State updates are lifecycle-bound to the instance, and automatically cleaned up during safeBox.close(), preventing leaks and ensuring proper teardown.

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Motivation

Previously, there was no way to observe what a SafeBox was doing at runtime unless you had external coordination. This addition makes it possible to:

• Display "in progress" indicators during writing.
• React to closure events in a scoped, low-overhead manner.
• Or simply track SafeBox behavior for close observations.

This is especially useful for ViewModel-scoped SafeBox instances that may be short-lived and tied to screen transitions.

API

SafeBox.create(
    context = ctx,
    fileName = "my_prefs",
    listener = SafeBoxStateListener { state ->
        when (state) {
            WRITING -> // do something
            IDLE ->    // do something
            CLOSED -> cleanUp()
        }
    }
)

Behavior Notes

• Listener receives the initial IDLE state upon creation.
• State changes:
  • WRITING during write() and delete().
  • IDLE once operations complete.
  • CLOSED when close() is called.
• No manual removal is necessary. Listener is instance-bound and disposed automatically during teardown.

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Partially addresses #12.
The remaining work (global observer support) will be completed in a follow-up PR.