docs: expand README and prepare 0.4.0 changelog

[tisonkun]

Summary

• expand the README with a getting-started example, sketch feature matrix, compatibility notes, and Apache community, security, and license guidance
• rebuild the Unreleased changelog from the final diff between 0.3.0 and main
• document the public migrations, features, and correctness fixes that users will observe while excluding internal work and APIs that were introduced and removed during 0.4.0 development

Changelog scope

The 0.3.0 tag is the comparison baseline. Entries describe the final 0.4.0-facing behavior rather than the sequence of implementation changes that led to it.

Validation

• cargo x prepare-testdata
• cargo x check
• cargo x test
• cargo x lint
• README external links checked