fix(runtime): deliver JS process stdout/stderr to a redirected fd (pipe/file)

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Guest process.stdout/process.stderr (and console.log/error) wrote via the _log/_error bridge globals, which append to the process's captured output and bypass kernel fd 1/2. So any redirection was silently dropped: 'node -e ... | cat' produced empty output (wc -c saw 0 bytes) and 'node -e ... > file' wrote nothing, while fs.writeSync(1, ...) — which goes through the kernel fd — worked. Confirmed pre-existing on 0.2.20-rc.1.

Route _stdout/_stderr through the existing _kernelStdioWriteRaw bridge (__kernel_stdio_write -> write_process_stdout/stderr on kernel fd 1/2), the same path WASM stages use, so redirection and pipes are honored. Falls back to _log/_error only when the kernel bridge is unavailable (e.g. browser). Top-level captured output is unchanged (the sidecar drains fd 1).

Verified: process.stdout|cat, console.log|cat, node|wc -c (6 bytes), node>file, console.error 2>&1|cat all deliver output; top-level node output and the #1959 pipelines are unaffected. Note: the symmetric stdin-stream gap (process.stdin does not emit 'data' from a piped fd 0, though fs.readSync(0) works) is a separate follow-up.

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Stack for rivet-dev/agentos

• #1967 fix(runtime): deliver JS process stdout/stderr to a redirect … 👈
• #1966 fix(runtime): a blocking WASM pipe read waits for its writer …
• main

Get stack: forklift get 1967
Push local edits: forklift submit
Merge when ready: forklift merge 1967

_{change rsnpzpss}