chore: 0.2.0 publish prep — package hygiene and loom as a dev-dependency

[hasanzakeri]

Pre-publish cleanup for 0.2.0, found while reviewing cargo publish --dry-run.
Three independent changes, one per commit.

1. docs(changelog) — date 0.2.0 to its actual release date

The section was dated 2026-06-12, when the work was authored. The release ships today, and that date travels inside the published tarball.

2. chore(package) — keep Makefile and .gitignore out of the crate

Contributor-workflow files with no meaning to a consumer. Packaged file count 28 → 26.

3. refactor(loom) — make loom a dev-dependency

The substantive one. As a [target.'cfg(loom)'.dependencies] entry, loom was resolved into the lockfile of every crate depending on rt-ring. Measured with a throwaway crate depending on rt-ring:

	packages in downstream Cargo.lock
before	32 — loom, generator, the tracing-subscriber tree, cc, windows-sys, …
after	2 — the consumer and rt-ring

None of those were ever compiled — cfg(loom) is never set downstream — but they were recorded and fetched by cargo fetch, cargo vendor, and offline CI mirrors. Poor showing for a crate whose selling point is having no runtime dependencies; the README's claim is now literally true in a consumer's lockfile.

Dev-dependencies don't propagate to consumers. They also aren't linkable from the plainly-compiled library, and sync.rs needs loom::sync::atomic in the library itself. Unit tests are the exception — there the lib is compiled with --test, where dev-deps resolve — so the loom tests move from tests/ into src/loom_tests/. This is the arrangement tokio uses, for the same reason.

Git tracked both files as renames, so the move reviews as a diff.

Accepted cost: non-test builds under --cfg loom (cargo build, cargo clippy --lib) no longer resolve — off the --test compilation there is no loom to link, failing with E0433 at src/sync.rs:6. Documented in the CHANGELOG, src/loom_tests/mod.rs, and Cargo.toml. tokio has the identical limitation. CI is unaffected: the clippy job doesn't set the cfg, and the loom job now runs cargo test --lib.

Verification

• RUSTFLAGS="--cfg loom" cargo test --lib — 4 loom tests pass as loom_tests::spsc::* / loom_tests::broadcast::*
• cargo test --all-targets — 59 pass; cargo test --doc — 15 pass
• cargo clippy --all-targets -- -D warnings, cargo fmt --check — clean
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps — clean
• cargo publish --dry-run — clean, 27 files, 41.3 KiB compressed

Watch the Loom job. Its invocation changed (--test loom_spsc --test broadcast_loom → --lib) and that's the one change I could only exercise locally.

No public API change; 0.2.0 remains unpublished, so nothing here is breaking relative to a released version.