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>

CHANGELOG.md

@@ -112,6 +112,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- **The crate is renamed from `invoiceninjars` to `invoiceninja-sdk`**, to
+  match the repository it has always lived in
+  (`gitea.awkawb.cloud/awkawb/invoiceninja-sdk`). Depend on it as
+  `invoiceninja-sdk` in `Cargo.toml`, and import it as `invoiceninja_sdk`
+  (Cargo lowercases a package name's hyphens to underscores for the crate
+  identifier) — every `use invoiceninjars::...` becomes
+  `use invoiceninja_sdk::...`. Not yet published to crates.io, so this
+  lands before the name was ever claimed there.
 - `statics`, `search`, and `health_check` return concrete types instead of
   being generic over `T: DeserializeOwned`.
 - `send_email` now unwraps the `data` envelope, so `T` is the entity