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>

tests/real_shapes.rs

@@ -15,7 +15,7 @@ use std::collections::BTreeMap;
 use std::fs;
 use std::path::{Path, PathBuf};
 
-use invoiceninjars::models::*;
+use invoiceninja_sdk::models::*;
 use serde::{de::DeserializeOwned, Serialize};
 use serde_json::Value;