fix(node): fill child_process spawnSync result fields

[andrewtdiz]

Summary

• Fixes node:child_process — spawnSync result missing pid/signal/output/error fields #1936 by returning Node-shaped child_process.spawnSync result fields.
• Adds status, signal, output, pid, stdout, and stderr for completed children, including non-zero exits.
• Adds a command-not-found result shape with error.code === "ENOENT", status: null, signal: null, output: null, numeric pid, and undefined stdio fields.
• Adds a focused node-suite/child_process/sync/spawn-sync-result-fields parity fixture.

Reference

• Consulted locally, not committed: reference/deepwiki/deno-node-child-process-spawnsync-result-fields-2026-05-27.md.

Verification

• Baseline before fix: PERRY_NO_CACHE=1 ./run_parity_tests.sh --suite node-suite --module child_process --filter spawn-sync-result-fields failed because Perry printed keys: stdout,stderr,status where Node printed keys: status,signal,output,pid,stdout,stderr.
• PERRY_NO_CACHE=1 ./run_parity_tests.sh --suite node-suite --module child_process --filter spawn-sync-result-fields PASS.
• PERRY_NO_CACHE=1 ./run_parity_tests.sh --suite node-suite --module child_process PASS 2/2 after rebasing onto merged fix(node): throw for child_process sync failures #1949.
• PERRY_NO_CACHE=1 ./run_parity_tests.sh --filter test_parity_child_process PASS.
• cargo test -p perry-runtime child_process --lib PASS with pre-existing warnings.
• cargo fmt --all -- --check PASS.
• jq empty test-parity/known_failures.json PASS.
• git diff --check PASS.

Non-goals

• Default Buffer stdout/stderr or encoding option parity (node:child_process — exec/execFile/sync ignore the encoding option (no Buffer stdout/stderr) #1937).
• Sync non-zero throwing behavior (node:child_process — execSync/execFileSync do not throw on non-zero exit #1938 / fix(node): throw for child_process sync failures #1949).
• Async exec/execFile failure Error shape (node:child_process — exec/execFile failure Error missing .code/.signal/.killed/.cmd #1935).
• Timeout/maxBuffer errors, streaming subprocess IO, kill behavior, IPC/fork, or full signal-name parity.