feat: replace interprocess with Tokio native IPC

[Tunglies]

Scope

This draft tracks the kode-bridge 0.5.0 migration from interprocess to Tokio native Unix domain sockets and Windows named pipes.

Current phase

• Baseline commit only: cross-platform IPC integration tests and Criterion transport benchmark
• Production transport remains interprocess
• Candidate implementation will follow after the baseline CI result is recorded

Boundaries

• Do not merge yet
• No tag, release, or crates.io publish
• CI runner performance is informational; same-host macOS Criterion runs provide the before/after comparison