TOML array given for a scalar flag is silently coerced to a string

When flattening .ultra.toml, every scalar-section value is stringified with fmt.Sprint. A TOML array supplied where a scalar flag is expected (for example apps-dir set to a list) becomes the literal string "[x y]" and is fed to the flag with no error, so the flag silently takes a garbage value instead of the config being rejected.

A type mismatch on a typed flag (a string where an int is expected) does surface a clear error, so only the array-for-scalar case is silent. flatten should reject a non-scalar value for a scalar flag.

Start by locating the flatten function that processes scalar-section values in the Go package, then inspect the existing typed-flag mismatch handling. Add a regression test for an array supplied to a scalar flag, and consider the issue done when flatten rejects the non-scalar value with an error instead of stringifying it.

Written by the indexing model from the issue text.