release: prepare v0.4.0

[dinakars777]

Summary

This prepares the safety-focused v0.4.0 release. The crate version is bumped to 0.4.0, and the accumulated changelog entries are moved out of Unreleased under a dated release heading.

Testing

• cargo fmt --check
• cargo clippy -- -D warnings
• cargo test
• cargo build
• git diff --check
• cargo package --allow-dirty --list
• cargo publish --dry-run --allow-dirty
• cargo package --list
• cargo publish --dry-run