Sylir city sky v2: prepared panorama

• Source: sylir-city-sky-v1.png, the original built-in imagegen output. Its exact generation prompt is preserved in sylir-city-sky-v1.md.
• Preparation: FFmpeg, reproduced by node scripts/prepare-skybox.mjs from the app folder. Existing outputs are protected from overwriting.
• A symmetric smoothstep blend joins the outer 6% of each border; the central city is outside the repair region. The source is retained untouched.
• Lanczos resampling to 4096 × 2048 and a mild unsharp filter improve sampling, but do not add genuine detail absent from the original 1774 × 887 image.
• The imagegen seam-repair experiment was not used. The app uses this deterministic preparation of the original panorama.