Rename the crate from invoiceninjars to invoiceninja-sdk

The package name has never matched the repository it lives in
(gitea.awkawb.cloud/awkawb/invoiceninja-sdk, mirrored to
github.com/awkawb1/invoiceninja-sdk) — both pointed at `invoiceninja-sdk`
while `Cargo.toml` declared `invoiceninjars`. Lines everything up on the
name the repo has always used.

Cargo derives the crate's Rust identifier from the package name by
lowercasing hyphens to underscores, so every `use invoiceninjars::...`
becomes `use invoiceninja_sdk::...` — in doctests across `src/`, the four
examples, all four integration test files, and `README.md`. `Cargo.lock`'s
own self-entry picked up the new name on the next `cargo check`, which is
also when the rename gets its real check: every doctest and every test file
has to resolve the new path to compile at all, not just the ones a grep
happens to catch.

`flake.nix`'s `pname` and description follow, so `nix build` produces
`invoiceninja-sdk-0.1.0.drv` rather than a name that no longer matches
anything. `GOALS.md`'s two mentions — the hypothetical CLI's name and the
`cargo install` example — follow too, for the same reason.

Verified clean on both the dev toolchain and the 1.88 MSRV shell: full test
suite (209 tests), clippy, fmt, and `nix build`/`nix flake check`.

Not yet published to crates.io (checked: the name 404s there), so this
lands before the old name was ever claimed — no one's `Cargo.toml` breaks
today, but it's recorded as a Changed entry since `GOALS.md` asks for
breaking changes to be legible in the changelog rather than discovered at
compile time, and the crate is reachable as a git dependency regardless of
crates.io status.

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

README.md

@@ -1,4 +1,4 @@
-# invoiceninjars
+# invoiceninja-sdk
 
 An async Rust client for the [Invoice Ninja](https://www.invoiceninja.com/) v5 API.
 
@@ -33,7 +33,7 @@ nix develop .#msrv --command cargo test
 ## Usage
 
 ```rust
-use invoiceninjars::{InvoiceNinjaClient, models::ListParams};
+use invoiceninja_sdk::{InvoiceNinjaClient, models::ListParams};
 
 // Hosted app:
 let client = InvoiceNinjaClient::hosted(std::env::var("IN_API_TOKEN")?)?;
@@ -75,7 +75,7 @@ with exponential backoff. Both are configurable:
 
 ```rust
 use std::time::Duration;
-use invoiceninjars::RetryPolicy;
+use invoiceninja_sdk::RetryPolicy;
 
 let client = InvoiceNinjaClient::hosted(token)?
     .with_timeout(Duration::from_secs(300))
@@ -164,7 +164,7 @@ only way the API creates one. Sixteen resources support it, as
 `upload_client`, `upload_invoice`, `upload_expense`, and so on:
 
 ```rust
-use invoiceninjars::FileUpload;
+use invoiceninja_sdk::FileUpload;
 
 let receipt = FileUpload::new("receipt.pdf", std::fs::read("receipt.pdf")?)
     .mime("application/pdf");