reapplyInvisibilityIfOurs

Re-applies the invisibility potion effect if this disguise originally applied it. Called from a PlayerRespawnEvent listener — vanilla Minecraft clears all potion effects on death, so a disguised player who died would respawn visible (the disguise itself persists per design, but the invisibility is gone). No-op if this disguise didn't add the effect in the first place (e.g. the player already had it from another source).