🐛 wrap non-Error causes passed to Err()

[rauhryan]

Motivation

Err() currently accepts Error, but JavaScript catch blocks and promise rejections can produce unknown thrown values. That makes it awkward to pass caught values directly to Err() and can leak non-Error throw values through APIs that model failures as Error.

This splits the Err() behavior change out so it can target the same base branch as #1149.

Approach

• change Err() to accept unknown
• preserve existing Error instances as-is
• wrap non-Error causes in ThrowValueError(String(cause), { cause })
• remove internal as Error casts where caught values are passed to Err()
• add regression coverage for Err() and until(Promise.reject(...))

[cowboyd]

This has been superseded by #1149