child_process: fix custom stdio pipe writable property

[mhayk]

Problem

When a parent process spawns a child with custom stdio pipes (stdio index >= 3), those pipes are created as readable-only instead of writable. This prevents the parent from writing to the child's custom pipes and causes ERR_STREAM_WRITE_AFTER_END errors.

Root Cause

The socket creation logic simplified the readable property calculation to i > 0, treating all non-stdin streams as readable. This is correct for stdout/stderr (parent reads from child) but incorrect for custom pipes (parent writes to child by default).

Solution

Pass the stdio index to createSocket() and determine socket readability based on the index:

• stdin (0): readable=false, writable=true (parent writes)
• stdout/stderr (1,2): readable=true, writable=false (parent reads)
• custom pipes (3+): readable=false, writable=true (parent writes by default)

Testing

• New regression test: test-child-process-custom-pipe.js ✅
• All 110 child_process tests pass ✅
• All stdin-related tests pass ✅

This is a pre-existing bug fix without a tracked issue number.