fix(node): honor child_process exec buffer encoding

[andrewtdiz]

Fixes part of #1937.

Summary

• Honor async child_process.exec() / execFile() encoding: "buffer" and encoding: null for callback stdout/stderr values.
• Route those outputs through Perry's existing Buffer allocator while preserving the default string output path.
• Add a focused node-suite fixture covering exec(..., { encoding: "buffer" }), execFile(..., { encoding: null }), and default string output.

Baseline

• Node passed Buffers for stdout/stderr under encoding: "buffer" and encoding: null.
• Perry previously passed strings, so Buffer.isBuffer(stdout) was false and stdout.toString("hex") returned the original text instead of hex bytes.

Reference

• DeepWiki comparison saved locally only at reference/deepwiki/deno-node-child-process-exec-encoding-buffer-2026-05-27.md.
• Extracted invariant: default output is UTF-8 strings; encoding: "buffer" or null produces Buffer stdout/stderr; exec shares execFile encoding behavior.

Tests

• PERRY_NO_CACHE=1 ./run_parity_tests.sh --suite node-suite --module child_process --filter exec-encoding-buffer
• PERRY_NO_CACHE=1 ./run_parity_tests.sh --suite node-suite --module child_process
• PERRY_NO_CACHE=1 ./run_parity_tests.sh --filter test_parity_child_process
• cargo test -p perry-runtime child_process --lib
• cargo fmt --all -- --check
• jq empty test-parity/known_failures.json
• git diff --check

Non-goals

• Sync execSync / execFileSync return ABI changes.
• spawnSync Buffer-vs-string result output shape.
• Named non-UTF8 encodings such as hex, unknown-encoding fallback behavior, or full encoding validation.
• Promisified exec output, timeout/maxBuffer handling, streaming subprocess IO, kill behavior, or IPC/fork.