[rupayon123]

When the prompt scanner reaches clean EOF, it sends an empty answer with a nil error and leaves its channel open. A subsequent Prompt therefore invents a default answer; later calls wait for cancellation even though the input stream has ended.

Close the scanner-owned channel on completion, send only real scanner errors, and return io.EOF when the cached channel is closed. Blank lines still select the configured default, and an unterminated final line is delivered before EOF.

Go1.26.0/macOS arm64 validation:

• Five EOF regressions fail on the base and pass after the fix.
• Full suite:257 passing leaf cases across16packages; affected subtree race:110 passing cases across five packages. Vet, build, gofmt, tidy/module locks and Staticcheck pass.
• A native subprocess with actual piped stdin returns Ada followed by three EOF results; the base invents a fallback and then times out.
• Supplementary full race testing still fails in unchanged progress.TestMultiProgressAutoRefreshMarksDirty. That same failure is reproduced narrowly on the pristine base; no progress changes or test waiver are included.

The existing scanner token limit, per-reader cache and input ownership remain unchanged. Hosted OS/runtime and terminal-event backends are separate from this stream validation.