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Add an opt-in --stdout-buffering build option for line-heavy stdout - #2960
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A build that sets it keeps console and process.stdout bytes in the runtime's 64 KiB buffer when stdout is not a terminal. The buffer settles when it fills, before any stderr write, when the event loop sleeps, before a blocking stdin read, before process.kill, on process.exit and on an uncaught error, and at exit; a terminal keeps one write per call. Builds without the option are unchanged.
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--stdout-buffering settled buffered stdout only in the event loop's poll arm, so a loop waiting on a timer or on a child slept with lines still in the buffer. The settle now runs before whichever wait arm the loop takes. Buffered bytes also move from the C stream buffer to a runtime-owned buffer submitted through the loop that retries EINTR and continues short writes, because fflush(3) drops the stream's remaining bytes when a handled signal interrupts a write that transferred nothing. Every place the runtime flushed the C stream to publish stdout now settles that buffer first, so process.exit, an uncaught error, a warning, a child inheriting stdout and the exit hook all publish what the program wrote, and stdout keeps its order against stderr on a shared descriptor.
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scriptc build --stdout-bufferingkeeps console andprocess.stdoutbytes in the existing 64 KiB buffer when stdout is not a terminal; a terminal keeps one write per call, and builds without the option are unchanged.process.kill, onprocess.exitand on an uncaught error, so2>&1order and prompt-then-read match Node; a SIGKILL can lose up to 64 KiB.console.loglines to a pipe: 0.54 s with the option, 6.3 s without, 19.0 s in Node 24 (11.8x faster than the default build); fasta n=1M 11.2% faster (13.4% on the ABBA arm means), log-lines 3.35x faster.