perf(ci): reuse build results across workflows

[donbeave]

Summary

Make GitHub-hosted runners the default jackin❯ lane while preserving explicit Velnor and combined execution. Reuse prepared Rust tools, registry state, and exact crate results so clean GitHub runs avoid repeated downloads and compilation. Keep publication single-writer when both lanes run.

What ships

• GitHub is the omitted/default lane for CI, Docs, Construct, Preview, Release, and jackin-dev workflows.
• Dispatch inputs accept github, velnor, and both; both retains one artifact writer.
• GitHub jobs use pinned Ubuntu 26.04 labels.
• Prepared xtask/tool artifacts and registry caches are runner-neutral.
• Rust test jobs remain complete, independently readable crate jobs.
• Workflow concurrency and job timeouts bound redundant work.
• Hosted cross-validation installs Zig explicitly and uses a deterministic cargo-zigbuild wrapper.
• CI performance auditing accepts ANSI-bearing GitHub job logs.

Verification evidence

Current head e299a42:

• actionlint .github/workflows/ci.yml
• cargo test -p jackin-xtask: 278 passed
• cargo fmt --check
• git diff --check
• GitHub Actions required checks are the merge gate.

Deviations

Mutation-only publication and cleanup workflows remain single-writer. They do not duplicate writes under both.

Not included

Remote sccache and RFC3161 attestation remain separate work.

Verify locally

actionlint .github/workflows/*.yml
cargo fmt --check
cargo clippy -p jackin-xtask --all-targets -- -D warnings
cargo nextest run -p jackin-xtask
cargo run -p jackin-xtask --locked -- lint ratchet

Migration notes

None.