chore: prepare dhttp v0.3.0 release

[eareimu]

Summary

• bump dhttp facade and SDK package versions to 0.3.0
• update h3x to 0.5.0 and dyns to 0.5.0
• bump dhttp-home to 0.3.0, remove the legacy home-loading API, and keep scoped home loading as the public entrypoint
• keep dhttp-identity and dhttp-access on 0.2.0 because their own public versions did not change
• improve crates.io publish workflow validation

Validation

• cargo fmt
• cargo test --workspace --all-targets --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• cargo semver-checks for dhttp 0.2.0 -> 0.3.0
• cargo semver-checks for dhttp-home 0.2.0 -> 0.3.0

Release notes

• release tag planned as v0.3.0
• waits for h3x 0.5.0 and dyns 0.5.0 registry visibility before final publish readiness