render: capture display-list fixtures on request

[dcvz]

Set FAST3D_CAPTURE_DIR and FAST3D_CAPTURE_FRAMES=60,250 and helix writes one .f3dcap per listed frame serial, recorded inside the blocked-guest consume so the snapshot is consistent. Needs fast3d with the capture feature (retrofoundry/fast3d-rs#27); merge that first.

[dcvz]

Superseded by #37. This branch predates the fallible memory API that landed in #36, so it no longer compiles and rebasing it isn't worth it — #37 ports the same hook onto main instead, keeping FAST3D_CAPTURE_DIR/FAST3D_CAPTURE_FRAMES and the same frame-%06d.f3dcap output, and adds sequence recording on top.

Two things changed in the port: serial selection is one-based now so it matches the filenames it writes, and an empty element in the serial list no longer silently turns capture off.