Add spec-specific HoverCast actions

[dfrisone]

What does this PR do?

Adds Target Unit and Context Menu to HoverCast's spec binding picker. A bound, enabled spec action replaces its global action for that specialization; clearing, disabling, or gating the spec action restores the global binding.

How was it tested?

• Tested in-game: added action overrides and switched between healer and DPS specializations.
• Isolated binding regression coverage: global fallback, spec switching, disabled/unbound/context-gated overrides, conflict handling, and complementary spell bindings.
• Lua syntax and repository style checks passed.

Screenshots

Not available.

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