Initial commit: async Rust client for the Invoice Ninja v5 API

Covers Clients, Invoices, Quotes, Credits, Payments, Products, Expenses,
Projects, Tasks, and Vendors, each with list, list_all, get, create,
update, and delete, generated by the resource_methods! macro.

Every model carries a #[serde(flatten)] extra map so fields the API
returns but the crate does not model survive a get/update round-trip
instead of being silently dropped.

Includes a Nix dev shell (Rust via rust-overlay), a GitHub Actions CI
workflow running build/test/fmt/clippy, and round-trip tests for the
extra-preservation invariant.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/lib.rs

@@ -0,0 +1,22 @@
+//! An async Rust client for the [Invoice Ninja](https://www.invoiceninja.com/) v5 API.
+//!
+//! ```no_run
+//! use invoiceninjars::{InvoiceNinjaClient, models::ListParams};
+//!
+//! # async fn run() -> invoiceninjars::Result<()> {
+//! let client = InvoiceNinjaClient::new(
+//!     "https://invoicing.co/api/v1",
+//!     std::env::var("IN_API_TOKEN").unwrap(),
+//! )?;
+//!
+//! let clients = client.list_clients(&ListParams::new().per_page(50)).await?;
+//! # Ok(())
+//! # }
+//! ```
+
+mod client;
+mod error;
+pub mod models;
+
+pub use client::InvoiceNinjaClient;
+pub use error::{ApiErrorBody, Error, Result};