AtlasSpigot 26.2 - v5 (config file renames + deployment fix)

Change from v4 - this one has real source changes, not just config:

• purpur.yml renamed to atlas.yml, config/leaf-global.yml renamed to config/atlas-global.yml (Main.java CLI default + LeafConfig.java constants). Verified by booting with an old leaf-global.yml deliberately planted alongside the new jar - it was correctly ignored, confirming the rename is real, not cosmetic.
• Fixed start.sh, which still referenced a stale jar filename from an earlier round - a bug in this repo, not something you reported, caught while making this change.
• README now explicitly documents the actual root cause behind the "F3 branding is gone" report: the config files never made it onto that deployment, only the jar did, so everything silently fell back to defaults with no error. Also documents the EULA requirement and that first-boot spawn-chunk generation time is a one-time cost that scales with the host's CPU allocation (a single-vCPU container will always take longer here, that's not fixable in the jar).

See README for the full setup steps and the complete config list.