perf: buffer native stdout writes

[He-Pin]

Motivation:

Scala Native kube-prometheus rendering still showed write/output overhead after the renderer and strict JSON import stack. NativeOutputStream already bypasses the JVM-compatible PrintStream path by writing through C fwrite, but stdout still used the platform default stdio buffering.

Key Design Decision:

Keep this optimization Native-only and local to stdout buffering. Instead of changing renderer flush thresholds or ByteBuilder behavior globally, configure the C stdio stream with full buffering before any NativeOutputStream writes occur. Passing a null buffer lets libc own the buffer lifetime, so the Scala object does not need to retain native memory.

Modification:

• Configure NativeOutputStream with setvbuf(file, null, _IOFBF, 256 KiB) during construction.
• Leave JVM, JS, YAML, expect-string, and file-output code paths unchanged.
• Preserve existing explicit flush() behavior for trailing newline and close handling.

Benchmark Results:

Workload: jrsonnet/tests/realworld/entry-kube-prometheus.jsonnet -J vendor

Candidate was benchmarked on the Scala Native 0.5.12 stacked exploration branch against clean cf7b8af9.

Order	Clean	Candidate	Result
Forward mean	218.848 ms	188.528 ms	-13.9%
Forward median	215.517 ms	187.368 ms	-13.1%
Reverse mean	224.045 ms	183.701 ms	-18.0%
Reverse median	224.281 ms	182.914 ms	-18.4%

Output equality matched by cmp.

Validation:

• ./mill --no-server --ticker false --color false __.reformat
• ./mill --no-server --ticker false --color false -j 1 __.test — 444 passed, 0 failed
• ./mill --no-server --ticker false --color false bench.runRegressions

Analysis:

This is a lower-risk write/flush optimization than increasing ByteBuilder thresholds: it does not alter rendering order, JSON escaping, object materialization, or JVM/JS behavior. It only changes the buffering policy of the Native stdout FILE*, and explicit flushes still happen at the same public boundaries.

References:

• Scala Native 0.5.12 migration PR: chore: upgrade Scala Native to 0.5.12 #867
• Related performance stack context: perf(stdlib): skip getBytes(UTF_8) for ASCII-safe base64 inputs #863, perf: bulk-write safe runs in BaseRenderer.escape #864, perf: hand-rolled YAML quote scanner removes fastparse allocations in Native #865, perf: skip UTF-8 encode for clean-ASCII long strings in renderer #866, fix: sort ASCII-safe strings by runtime kind #868

Result:

Native stdout rendering writes fewer/smoother buffered chunks for large JSON output while preserving byte-identical output and the existing flush contract.