Adds three structural assertions that run after the zip is built:
1. No backslash separators in any entry name. NeoForge needs
"META-INF/neoforge.mods.toml" to be at exactly that forward-slash
path; PowerShell 5.1's [ZipFile]::CreateFromDirectory() puts
"META-INF\neoforge.mods.toml" instead, which the loader silently
rejects. The current build code uses CreateEntry with explicit
forward slashes, but this guard fires if anyone reverts to the
simpler-looking CreateFromDirectory.
2. META-INF/neoforge.mods.toml exists at the canonical path. Without
it, NeoForge skips the jar with a bland "not a valid mod file"
warning that's easy to miss in a 500-line game log.
3. The modId declared in the embedded TOML matches the source folder's
modId. Catches the case where a tweak folder is renamed but its
neoforge.mods.toml isn't updated, which would otherwise ship a jar
whose declared identity differs from its filename.
Each tweak jar's build line now tags "[validated]" so a casual reader
of the log sees that the post-build checks ran. Failure raises a
specific exception with the offending entries listed, so the build
fails loudly at the source instead of producing a pack that mysteriously
doesn't apply its tweaks at runtime.
Matt Sijbers
372b5090cd