Does Singularity trigger a showdown, and can opponents respond with action cards like Primal Strength?

Ruling: Singularity does not trigger a showdown, but it does start a chain. Opponents cannot respond with action cards like Primal Strength, only reaction cards can be played in response.

Sequence:

• Singularity is played and starts a chain
• Opponents can only respond with reaction speed cards
• Action cards cannot be played because no showdown is triggered
• The chain resolves

Nuances:

• Showdowns only occur when units are moved to battlefields (occupied or unoccupied)
• Action spells can only be played outside of showdowns or inside of one, not in response to non-showdown chains
• Action spells can only start a chain, while reaction spells can start a chain or be added to an existing chain