Add ergonomic typed extraction and rewrite APIs

[bakgio]

Summary

This PR adds a thin ergonomic API layer on top of the existing low-level mp4forge primitives for common path-based extraction and rewrite workflows.

What changed

• Added typed extraction helpers:
  • extract_box_as
  • extract_boxes_as
  • extract_boxes_as_with_registry
• Added typed rewrite helpers:
  • rewrite_box_as
  • rewrite_boxes_as
  • rewrite_boxes_as_with_registry
• Improved extraction and rewrite diagnostics to include matched path, box type, and byte offset for payload failures
• Added high-level examples for the new helper layer while preserving the existing low-level examples
• Added integration coverage for typed success paths, passthrough behavior, and contextual error reporting
• Polished public docs and README coverage for the new helper surface
• Bumped the crate to 0.2.0 and updated release-facing metadata and changelog
• Added semver checking to CI as a release gate

Why

The low-level API was already solid, but common read/edit flows still required too much boilerplate. These helpers keep the existing architecture intact while making typical typed workflows easier to write, easier to read, and easier to debug.