Notify client state subscribers on loading transitions

[Junnyyy]

TL;DR

Notify subscribeToClientState() listeners when load() and reload() enter loading, while keeping flag-value subscriptions unchanged.

What changed?

• Updated core client loading flow to emit client state changes when a fetch starts and when it settles
• Preserved subscribe() and subscribeToFlag() behavior so they still only fire on effective flag value changes
• Added a regression test covering the loading transition and subscriber separation
• Updated the expected client-state notification count in the existing override flow test

How to test?

• Run the core test suite and confirm the new loading-transition coverage passes
• Verify the regression test asserts subscribeToClientState() sees loading before the fetch resolves
• Confirm flag subscribers do not fire for loading-only transitions and still fire when values change

Why make this change?

• Devtools and inspection surfaces need immediate loading/status updates without broadening normal flag subscriptions
• This keeps client-state metadata responsive while preserving the existing semantics of value-based subscriptions

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Note

Low Risk
Narrow subscription-behavior change in core with tests; no auth or data-path changes, though devtools or inspection UIs may see extra client-state callbacks.

Overview
load() / reload() now emit subscribeToClientState() callbacks as soon as status becomes "loading", not only when the fetch finishes or errors. subscribe() and subscribeToFlag() are unchanged and still run only when effective flag values change.

AGENTS.md documents this split. Core tests add a regression case for the in-flight loading notification and update an existing client-state notification count.

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