release: stage dhttp-identity v0.2.0

[eareimu]

Staged member-crate release prep

• Stages dhttp-identity v0.2.0 as the bootstrap crate for the broader DHTTP release wave.
• Includes the certificate-chain sequence constraint change that downstream h3x/ddns/dhttp release prep expects.
• Keeps the rest of the dhttp workspace on v0.1.0 for this staged branch so this PR is scoped to the member crate only.

Release gates

• This PR prepares the exact commit that can be used for the user-owned/manual dhttp-identity v0.2.0 bootstrap publication if approved.
• It does not replace the full dhttp v0.2.0 release PR (release: prepare v0.2.0 #3), merge gate, final registry convergence, or final tag.
• Merge/tag/publish remain blocked on explicit user approval and the broader release/genmeta-test gates.

Verification

• cargo metadata --manifest-path /home/yiyue/code/reimu/dhttp/Cargo.toml --no-deps --format-version=1
• cargo test -p dhttp-identity --all-features
• cargo publish -p dhttp-identity --dry-run --allow-dirty
• cargo test --workspace --all-features --all-targets
• cargo fmt --all -- --check
• git diff --check