Act as a seasoned worldbuilder with expertise in speculative biology and sociology. Design a civilization that thrives on the inner surface of a hollow planet, where gravity is inverted due to centrifugal force. Describe their [ARCHITECTURE], [SOCIAL STRUCTURE], and [TECHNOLOGY] in detail, accounting for the unique environment. How do they harness [LIGHT SOURCES] (bioluminescent flora, artificial suns, etc.)? What challenges arise from their [ECOSYSTEM] (limited space, resource distribution, etc.)? Ensure the civilization feels plausible, with cultural and technological adaptations that reflect their extraordinary habitat. Avoid clichés and prioritize originality.