Handle ShaguTweaks readiness consistently

[Hippoom]

Summary

• Add shared ShaguTweaks readiness handling for API-ready and config-ready states.
• Update Shagu-dependent Reforged modules to use the same readiness model.
• Fix load-order cases where ShaguTweaks loads before Reforged modules register their own events.

Details

ShaguTweaks can load before or after Reforged depending on addon load order and client setup. Previously, different Reforged modules checked for ShaguTweaks readiness in different ways. Some modules could miss the ADDON_LOADED event if ShaguTweaks had already loaded, while the GUI could build before ShaguTweaks_config was available.

This PR adds a shared readiness helper with two levels:

• Shagu API ready: ShaguTweaks, ShaguTweaks.T, and ShaguTweaks.mods are available
• Shagu config ready: Shagu API is ready and ShaguTweaks_config is available

The cast icon and Shagu GUI now use this shared readiness path instead of local timing logic.

Test plan

• Tested in-game with ShaguTweaks installed.
• Verified the player cast icon appears when "Show Icon" is enabled.
• Verified the Reforged ShaguTweaks panel populates instead of showing only the reload UI button.
• Verified behavior remains stable across reloads during local testing.

Fixes #15