release: prepare v0.4.0

[eareimu]

Release prep

• Bumps h3x to v0.4.0.
• Moves the dhttp-identity edge to v0.2.0 through the dhttp release-prep branch while the staged member crate is not yet on crates.io.
• Uses v0.4.0 rather than a v0.3.x patch because h3x publicly exposes dhttp_identity authority types and traits.

Release gates

• Wait for the staged dhttp-identity v0.2.0 publication/visibility gate.
• Replace the provisional git dependency with the final crates.io dependency before merge/tag once the upstream version is visible.
• Merge, tag, and publication remain blocked on full genmeta-test completion and explicit user approval.

Verification

• cargo metadata --manifest-path /home/yiyue/code/reimu/h3x/Cargo.toml --no-deps --format-version=1
• cargo +nightly fmt -- --check
• git diff --check

Full clippy/test and publish dry-runs are intentionally pending until the same-wave dependency visibility gates are cleared.