build 0.1.21 — Once Upon The Universe Steam update #1329018

Reworked sector metallicity into a scientifically grounded Zsun scale, with updated thresholds for planet formation, biospheres and life-related progression while preserving the intended gameplay pacing. Rebalanced planet-star collisions: swallowed rocky planets now barely increase stellar mass, enrich future supernovas, and can trigger heavy-particle overload instability. Improved first civilization tutorial guidance by centering and highlighting the inhabited planet after the double-click instruction. Improved star system spacing to reduce overlapping planetary orbits and accidental planet collisions between neighboring systems. Added drifting matter generation.