chore: dual-license MIT OR Apache-2.0

The Rust ecosystem convention, and what kickscore, xy and saphyr already use —
kickscore being the closest sibling to this crate.

LICENSE-APACHE is the canonical 201-line Apache-2.0 text with the appendix
left as the unfilled template, which is the form Rust crates ship. LICENSE-MIT
carries the copyright line. Both are picked up by cargo automatically and
appear in the packaged crate.

This also unblocks the release workflow. `cargo publish` does not check the
license field when the target is an alternative registry, but cargo-release
does, and refuses outright:

    error: trueskill-tt is missing the following fields:
             license || license-file

--no-verify does not bypass it. So `just release` was inert without this,
whatever cargo publish alone would have accepted.

README gains the conventional dual-license section and the contribution note
that dedicates inbound contributions under the same terms.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T5SYDExxL4vZgvunrcNSMc

README.md

@@ -101,3 +101,20 @@ h.converge().unwrap();
 - [x] Add Observer (`Observer` / `NullObserver`)
 - [x] Benchmark the inference loop (`benches/batch.rs`, `benches/history_converge.rs`, `benches/ingest.rs`)
 - [ ] Cross-check `quality()` against [sublee/trueskill](https://github.com/sublee/trueskill/tree/master) — N-group support works and is covered by invariants, but no reference values are asserted
+
+## License
+
+Licensed under either of
+
+- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
+  <http://www.apache.org/licenses/LICENSE-2.0>)
+- MIT license ([LICENSE-MIT](LICENSE-MIT) or
+  <http://opensource.org/licenses/MIT>)
+
+at your option.
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.