fix(child_process): support stream-backed stdio

[andrewtdiz]

Summary

• Recognize opened fs.ReadStream/fs.WriteStream entries in child_process stdio arrays and route them through the existing fd-backed stdio path.
• Preserve Node's fd-backed surface shape for these entries: child stdio properties and captured sync result slots become null while bytes go to the supplied files.
• Add Node parity coverage for async spawn/fork and sync spawnSync/execSync/execFileSync stream-backed stdio.

Issue Cluster

Refs #2555.
Refs #2130.

This is batched as one slice because async spawn/fork and sync helpers share cp_read_stdio, and the implementation can reuse the existing fd routing path once fs stream objects are normalized to registered fds.

Validation

• CARGO_TARGET_DIR=/tmp/perry-child-stream-stdio-check cargo check -p perry-runtime
• cargo build -p perry-runtime -p perry --release
• cargo fmt -p perry-runtime -- --check
• git diff --check
• git diff --cached --check
• ./scripts/check_file_size.sh
• jq empty test-parity/known_failures.json
• PATH=/root/.npm/_npx/bac97da9607b7ef2/node_modules/node/bin:$PATH PERRY_NO_AUTO_OPTIMIZE=1 PERRY_NO_CACHE=1 ./run_parity_tests.sh --suite node-suite --module child_process --filter stdio-stream
• Adjacent stdio filters: stdio-fd, stdio-ignore, stdio-inherit, spawn-sync-stdio, child_process/sync/sync-stdio
• Full child_process parity module: 26 pass, 0 fail, 0 compile fail, 0 skipped

Note: cargo fmt --all -- --check currently reports an unrelated formatting diff in crates/perry-stdlib/src/webcrypto/jwk.rs:494; this PR keeps that file untouched and uses the package-scoped perry-runtime format check for the touched Rust code.

Non-Goals

• Arbitrary custom stream objects without registered file descriptors.
• Socket/TTY handle passing or broader libuv-style handle semantics.
• Invalid or not-yet-open stream validation edge cases.
• Other node:child_process: honor spawn and exec option semantics beyond cwd/env/shell #2555 options such as AbortSignal, killSignal, uid/gid, detached, and platform-specific flags.