feat(dist): crates.io + binstall + Windows install.ps1 + license files

Distribution prep on the road to public availability (plan steps 2–3a).

- Cargo.toml: publish-ready (drop publish=false; homepage/keywords/
  categories/exclude) + [package.metadata.binstall] with per-target
  overrides (linux-gnu->musl, windows-msvc->gnu/gnullvm). dry-run clean.
- scripts/install.ps1: Windows `irm | iex` one-liner — written but
  untested here (no PowerShell; validate on Windows). README Windows block.
- README.md (new); LICENSE-MIT + LICENSE-APACHE (dual, (c) Lazy
  Evaluation Ltd); CONTRIBUTING.md (inbound=outbound dual-license note).
- ADR-0055 Amendment 1 (install.ps1), ADR-0056 (crates.io/binstall),
  README index + plan updates.

The actual `cargo publish` remains a gated maintainer step (token,
irreversible) at a new tagged release; real cargo-binstall validation
pending.

CONTRIBUTING.md

@@ -0,0 +1,20 @@
+# Contributing to rdbms-playground
+
+Contributions are welcome — bug reports, ideas, and pull requests. The
+project lives on Gitea at
+<https://git.lazyeval.net/oli/rdbms-playground>; please file issues and
+open pull requests there. It's approaching its first public release, so
+the most useful contributions right now are bug reports and rough edges
+you hit while learning.
+
+## License of contributions
+
+Unless you explicitly state otherwise, any contribution you intentionally
+submit for inclusion in this project — as defined in the Apache-2.0
+license — shall be **dual-licensed under `MIT OR Apache-2.0`** (the
+project's licenses), without any additional terms or conditions.
+
+This is the standard Rust "inbound = outbound" arrangement: your
+contribution is offered under the same licenses the project distributes
+under, so — via Apache-2.0 §5 — it carries the Apache-2.0 §3 patent grant
+to all users. No separate CLA is required.