ReadFrom: keep the bytes returned together with a read error

[youdie006]

io.Reader may return n > 0 together with a non-EOF error, and the caller should use those bytes before looking at the error. ReadFrom sets the error and breaks out without committing them or adding them to n (ring_buffer.go:553). So io.Copy(rb, src) from a source that ends with io.ErrUnexpectedEOF (a truncated HTTP body or flate stream) reports fewer bytes than it read, and the last chunk is lost. WriteTo already counts partial bytes on error since #20.

This commits and counts nr before setting the error. The errormock reader now returns its bytes together with the error, the way its Write already does, which makes three rows of TestRingBuffer_ReadFrom_Error fail on master; TestRingBuffer_ReadFrom_ErrorKeepsData also checks Length().

go vet, gofmt -s -l and go test -race ./... pass. One note on the last: TestRingBuffer_Copy hit its 60s timeout on my machine under heavy load, on master as well (3 of 3 isolated -race runs took 68-70s); its stream only returns nil or io.EOF, so it never reaches this branch. With it skipped the -race suite passes.

Written with AI assistance (Claude); I ran the checks above locally and reviewed the change.