feat(powerbars): add Boss Health Marker spark to UnitFrames and ResourceBars

[kylelemons]

What does this PR do?

Adds an opt-in Boss Health Marker toggle (bossPacingEnabled) to the Player Power Bar in both Unit Frames (Unit Frames > Main Frames > Power Bar) and Resource Bars (Resource Bars > Primary Bar).

During boss encounters, this renders a subtle 2px vertical sparkline on the player's power bar that aligns with boss1 current health percentage, assisting healers and DPS with resource and mana pacing against boss phases.

Key highlights:

• Zero Cost / Opt-In: Setting defaults to false across both modules.
• Strict Lazy Event Handling: Event frames (UNIT_HEALTH, UNIT_MAXHEALTH for boss1, INSTANCE_ENCOUNTER_ENGAGE_UNIT, PLAYER_REGEN_DISABLED/ENABLED) are only registered when bossPacingEnabled == true.
• Zero Taint / Secret Value Safe: The sparkline is parented to a 0-alpha StatusBar tracking UnitHealth("boss1"), using native UI frame alignment without running Lua math on protected combat values.
• Live Settings Preview: In EUI_UnitFrames_Options.lua, toggling the checkbox renders a live sparkline preview directly on the unit frame preview bar at the top of the settings page.
• Clean 2-Slot Layout: Paired cleanly with the specialization-dependent Power Type override in a unified DualRow with zero layout shift or vertical gutters.
• Local Chunk Compliance: Wrapped in a scoped do ... end block to respect Lua 5.1's 200 local variable limit.

How was it tested?

• Tested on retail client build 12.1.0.69587.
• Verified toggle defaults to OFF and produces 0 overhead when disabled.
• Tested during boss encounters in raid/dungeon; confirmed spark tracks boss health smoothly and hides outside of boss encounters.
• Tested live settings preview in UnitFrames options pane.

Screenshots

Type	Setting	Live
Unitframe
Resource Bar

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