Promoting raw BG3 gameplay bundle previews in the TD2 SDL port

Promoting raw BG3 gameplay bundle previews in the TD2 SDL port Today's checkpoint was small in code size but important in interpretation. The late gameplay bundles in the project already had useful BG1, BG2, OBJ, and screenshot-derived support surfaces, but they were still weak on one practical question: when design flagged the sky/horizon side of gameplay, were we looking at a missing asset, or were we looking at a composition problem? I closed that ambiguity by extending the gameplay bundle builder to emit first-class BG3 artifacts next to the existing layer outputs: bg3.ppm bg3.png bg3_render.json Then I refreshed the promoted late-entry bundles for frames 3250, 3400, and 3550. The useful result is that raw BG3 is clearly populated on these anchors. It carries the sky-to-horizon helper strip much more directly than BG2, which means the next renderer question is no longer "does this bundle even have usable BG3?". The next question is "which gating/composition rule makes that helper l