feat: Add Arena Number Names option

[Uberlicious]

What does this PR do?

In arenas it is very helpful to replace nameplate names with their respective numbers.

How was it tested?

Been using it for the last two days on live in arenas.

Screenshots

Disregard the healer icon on the Paladin, that's a separate addon.

Checklist

• New settings default OFF (no behavior change without opt-in)
• Zero cost while disabled: no events registered, no polling, no hooks doing work, no frames built
• Cheap while enabled: event-driven (no polling, no timer-based logic, no per-frame allocations)
• No writes onto Blizzard-owned frames (weak-table pattern used); HookScript/hooksecurefunc only, never SetScript on Blizzard frames
• Tested in-game on live; no version gates or pre-Midnight APIs added