fix: IterChan.Next returns true on error events without storing a new value

[childish-sambino]

When a walk worker sends on SendError, Next() stored the error and returned true without storing a new current value. Consumers call Resource() on every iteration (file/uploader.go:176, file/downloader.go:127), so an error event either panics on the nil interface conversion (error before the first file, e.g. a file deleted between ReadDir and stat) or silently processes the previous entry twice.

Next() now records the error for Err() and keeps waiting for the next value or Done, which is the contract consumers already assume (they check it.Err() after the loop). The walker cancels the iterator context on exit (defer it.Stop()), so skipping error events can't hang the loop.

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