Extension rail icons — flat set, v3 variants on top

Round 3. Feedback on the flat set: Custom Mockups' wide halo circle around the gear is a no-go; Pexels reads outline-thin next to the other solid brand marks (the official Pexels logomark is a solid tile with the P knocked out — the current icon uses only the P letterform from it). New variants below, everything previous kept underneath. Nothing gets deleted; older rounds stay reachable via the permalinks in the footer.