Summary

node --run <script> is a thin launcher: it never initializes V8/JS, it
just runs the package.json script command. Today it always spawns the
command through /bin/sh, keeps the (large) node process resident to
wait for the child, and then goes through the normal shutdown. That means
paying for a fork of the node process, an event-loop round-trip, and the
teardown of V8/ICU/OpenSSL static state — none of which the task runner
needs.

This PR makes the POSIX path hand the process off directly:

• Simple commands (a plain program args invocation with no shell
  metacharacters) are run directly via execvp(), skipping /bin/sh
  entirely. The augmented node_modules/.bin PATH and the NODE_RUN_*
  variables are applied first so resolution and the child environment are
  unchanged. Positional arguments after -- are passed through verbatim.
• Commands needing shell syntax (&&, |, $, globs, quotes, …)
  execve() /bin/sh -c <command>, still replacing the node process
  instead of forking a child and waiting.
• If the exec fails (e.g. the command is a shell builtin, or not found)
  it falls back to the original uv_spawn() path, so behavior is
  preserved for every case.

ICU initialization is also skipped for --run, since the Intl APIs are
never reached.

Windows keeps the existing spawn-based behavior.

Benchmark

macOS, node --run of a node_modules/.bin binary, interleaved A/B,
600 iters × 8 rounds:

The remaining time is essentially node's own load floor (node -v is
~12–13 ms of pre-main() dyld + V8/ICU static init), so this removes
most of the avoidable overhead between that floor and the script.

Behavior change worth a look

Because node now execs the target/shell directly, the script's exact
exit status is propagated (e.g. exit 2 → 2). The previous code
collapsed any non-zero status to 1. This matches npm run/shell
semantics; the existing test/parallel/test-node-run.js assertions
(codes 0 and 1) still pass, but flagging it explicitly.

Test plan

• test/parallel/test-node-run.js and
  test/message/node_run_non_existent.js pass (cover
  node_modules/.bin resolution, NODE_RUN_* env vars, parent-dir
  package.json search, and verbatim positional args — all now via the
  fast path)
• cpplint clean
• CI on Windows/Linux/macOS

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