f.readline(size) and open(..., errors="replace")

Vloot's ssh stream reader bounds each line from a peer it does not trust (proc.stdout.readline(MAX_LINE)) and opens /proc files with errors="replace". Reported by neo-73 2026-10-20 (readline(n) failed to compile with a ShortString endswith error).

• TPyFile.readline(size): at most size bytes, stopping after a newline. A negative size means no limit and 0 returns ''. The plain form calls readline(-1).
• open(..., errors="replace") is honoured, not just accepted. The frontend wraps the open in pyfile_errors_replace, and text reads (read, read(n), readline, readlines, iteration) decode invalid UTF-8 to U+FFFD.
• bytes.decode("utf-8", "replace"/"ignore"/strict) follows CPython's decoder: lead bytes C2..DF, E0..EF, F0..F4 only; the second-byte ranges that refuse overlongs and surrogates (E0 A0.., ED ..9F, F0 90.., F4 ..8F); one U+FFFD per maximal valid prefix.

Test: test_nilpy_file_readline_size_and_errors_replace (13 decode edge cases plus the file forms, python3's output).