Add optional ShaguTweaks target health values

[Hippoom]

Summary

• Add a Target frame text option for using ShaguTweaks real health values when available.
• Keep native UnitHealth / UnitHealthMax behavior as the default.
• Reuse ShaguTweaks libhealth data without re-enabling ShaguTweaks' conflicting health-number rendering module.

Details

Reforged disables ShaguTweaks' Real Health Numbers module because that module draws text onto the default Blizzard unit frames, which conflicts with Reforged's custom unit frames.

However, Reforged's target frame currently uses the native Vanilla API:

UnitHealth("target")
UnitHealthMax("target")

On Vanilla-like clients, these APIs can return percentage-style values such as 100/100 for hostile targets, even when ShaguTweaks has cached or estimated real mob health values through ShaguTweaks.libhealth.

This PR adds an opt-in Target frame text option:

Use ShaguTweaks real health values when available

When enabled, Reforged's target frame uses ShaguTweaks.libhealth:GetUnitHealth("target") if available, and otherwise falls back to native UnitHealth / UnitHealthMax.

Test plan

• Tested in-game with ShaguTweaks installed.
• Verified the option is disabled by default and native target health behavior is unchanged.
• Verified enabling the option shows real/estimated mob health values on OctoWoW instead of 100/100.
• Verified toggling the option updates target health text without a Lua error.

Fixes #17