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.

README.md

@@ -0,0 +1,98 @@
+# rdbms-playground
+
+A cross-platform terminal app for **learning relational database
+concepts** — tables, columns, primary and foreign keys, relationships,
+indexes, queries, and query plans — in a safe sandbox.
+
+It's a teaching tool, not a database administration tool. It meets
+beginners with guided, keyword-based commands (**simple mode**) and grows
+with them to raw SQL (**advanced mode**), so the same playground works
+from "what is a primary key?" through to writing real queries and reading
+their execution plans.
+
+Website & documentation: **<https://relplay.org>**
+
+## Install
+
+### One line (Linux / macOS)
+
+```sh
+curl -fsSL https://git.lazyeval.net/oli/rdbms-playground/raw/branch/main/scripts/install.sh | sh
+```
+
+Detects your OS and CPU, downloads the matching release binary, verifies
+its SHA-256 checksum, and installs it to `~/.local/bin`. Set
+`RDBMS_INSTALL_DIR` to install elsewhere, or `RDBMS_VERSION=vX.Y.Z` to
+pin a version. (Prefer to read before you run? The script lives at
+`scripts/install.sh`.)
+
+### One line (Windows, PowerShell)
+
+```powershell
+irm https://git.lazyeval.net/oli/rdbms-playground/raw/branch/main/scripts/install.ps1 | iex
+```
+
+Downloads the matching `.exe`, verifies its checksum, installs to
+`%LOCALAPPDATA%\Programs\rdbms-playground`, and adds it to your user
+PATH. Or use a package manager (Scoop / winget) once those land.
+
+### With `cargo binstall`
+
+If you have [`cargo-binstall`](https://github.com/cargo-bins/cargo-binstall)
+(install it first — it is not part of `cargo` itself):
+
+```sh
+cargo binstall rdbms-playground
+```
+
+### From source
+
+```sh
+cargo install rdbms-playground          # from crates.io
+# or, from a clone:
+cargo build --release                   # binary at target/release/rdbms-playground
+```
+
+### Prebuilt binaries
+
+Every release publishes static Linux, standalone Windows, and macOS
+binaries (x86_64 and aarch64) with `.sha256` checksums on the
+[releases page](https://git.lazyeval.net/oli/rdbms-playground/releases).
+Windows users can also use the binary directly (package-manager support
+is planned).
+
+## A quick taste
+
+```
+create table Customers with pk id(serial)
+add column Customers: name (text)
+add column Customers: email (text)
+insert into Customers values ('Ann', 'ann@example.io')
+show data Customers
+```
+
+Press **F1** while typing for a contextual hint about the command you're
+building, or type `help` for the full command list. Switch to raw SQL
+with `mode advanced` (or prefix a single line with `:`).
+
+## Project status
+
+Approaching its first public release. See the website for current
+features; installation via package managers (Homebrew, Scoop, winget) is
+on the roadmap.
+
+## License
+
+Dual-licensed under either of
+
+- MIT license ([LICENSE-MIT](LICENSE-MIT))
+- Apache License 2.0 ([LICENSE-APACHE](LICENSE-APACHE))
+
+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. See [CONTRIBUTING.md](CONTRIBUTING.md).