MidnightNameplates 0.9.5.1 - Phase 9.5.1 World Object Nameplate Filter

FIX
- Added a central IsEligibleNameplateUnit() gate.
- Real players are always eligible.
- Real NPC/creature units are accepted via creature type / classification signals.
- Object-style units without real creature/classification data are rejected.
- Mailboxes and similar interactable world objects are no longer skinned by MidnightNameplates.
- Eligibility runs before plate refresh/rule/threat/aura/cast presentation.

IMPORTANT
- This is not a hard-coded "Mailbox" name check.
- The filter is category-based so it also protects against other world-object nameplates.

TEST
1. Stand near a mailbox: MidnightNameplates should not draw a health bar on it.
2. Enemy NPC nameplates still work.
3. Friendly NPC nameplates still work if enabled.
4. Player nameplates still work.
5. Quest mobs still work.
6. Rules, casts, auras, threat, target marker and execute warning still work.
7. No Lua/taint/secret-value errors.
