chore: prepare 0.7.0 release

[tisonkun]

Summary

• Bump asyncband to 0.7.0 and refresh Cargo.lock.
• Finalize an undated v0.7.0 changelog section and leave Unreleased empty.
• Verify the undated heading before creating an RC and add the actual date through a post-release pull request.
• Freeze the release scope at fba31fd; open feature work remains deferred under Tracking Issue to Release 0.7.0 #227.

Validation

• cargo x check
• cargo x semver --release-version 0.7.0 --acknowledge-breaking-changes
• cargo x lint
• cargo x test --no-capture
• RUSTUP_TOOLCHAIN=1.86.0 cargo x test --no-capture
• cargo x miri
• cargo publish --package asyncband --locked --dry-run

Refs #227