Refine Rust xtask

Repository automation for the native Rust port belongs here: code generation, API contract export, fixture refresh, release packaging, installer smoke tests, and migration checks.

Commands:

• cargo run --manifest-path xtask/Cargo.toml -- api-contract
• cargo run --manifest-path xtask/Cargo.toml -- check-static-assets
• cargo run --manifest-path xtask/Cargo.toml -- runtime-layout
• cargo run --manifest-path xtask/Cargo.toml -- release-plan patch
• cargo run --manifest-path xtask/Cargo.toml -- release-check
• cargo run --manifest-path xtask/Cargo.toml -- test-unit
• cargo run --manifest-path xtask/Cargo.toml -- test-integration
• cargo run --manifest-path xtask/Cargo.toml -- test-rust
• cargo run --manifest-path xtask/Cargo.toml -- test-smoke-ai
• cargo run --manifest-path xtask/Cargo.toml -- test-cli
• cargo run --manifest-path xtask/Cargo.toml -- test-browser
• cargo run --manifest-path xtask/Cargo.toml -- test-cluster-ssh
• cargo run --manifest-path xtask/Cargo.toml -- test-install-uninstall
• cargo run --manifest-path xtask/Cargo.toml -- test-full-workflow
• cargo run --manifest-path xtask/Cargo.toml -- test-multi-instance-sync
• cargo run --manifest-path xtask/Cargo.toml -- test-all
• cargo run --manifest-path xtask/Cargo.toml -- check