fix: guard streaming stdout against binary chunks

[ryanfowler]

Summary

• continue binary detection across streamed stdout chunks
• preserve split UTF-8 sequences across read boundaries
• add unit and terminal coverage for mid-stream binary detection

Validation

• cargo fmt
• cargo test --locked --all-features --lib stream::tests::checked_copy -- --nocapture
• cargo test --locked --all-features --test terminal terminal_stdout_midstream_binary_detection -- --nocapture