Release mp4forge 0.3.0 with ergonomic byte helpers and BoxPath parsing

[bakgio]

Summary

This PR releases mp4forge 0.3.0 and includes the ergonomic follow-up work completed after 0.2.0.

What Changed

• added byte-slice convenience helpers for typed extract, rewrite, and probe flows
• added exact raw box-byte extraction helpers for full-box and payload-only reads, including registry-aware variants
• added additive BoxPath string parsing via BoxPath::parse, FromStr, and TryFrom<&str>
• added examples and test coverage for the new high-level ergonomic paths while preserving the existing low-level examples
• updated release-facing metadata and docs for the 0.3.0 release