release mp4forge 0.6.0

[bakgio]

Summary

• bump mp4forge from 0.5.0 to 0.6.0
• update release-facing version references in the crate metadata, README, lockfiles, and issue template
• add the 0.6.0 changelog entry covering the new async rollout and the related parity and concurrency work

Highlights

• add the additive Tokio-based async feature for seekable library I/O
• support async traversal, extraction, typed codec decode and encode, writer flows, rewrite flows, probe surfaces, and top-level sidx helpers
• make the supported async library paths usable with normal multithreaded tokio::spawn for independent-file workloads
• keep the CLI on the existing synchronous path and preserve the default non-async build
• lock sync-versus-async behavior with stronger parity, worker-thread, and concurrent-file coverage