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>

GOALS.md

@@ -90,7 +90,7 @@ Invoice Ninja account to contribute.
   ask "what does this endpoint actually return", which matters here because
   the e-invoicing and integration methods are typed as
   `serde_json::Value` and were verified against mocks rather than a live
-  service. It belongs in a sibling crate (`invoiceninjars-cli`) in a
+  service. It belongs in a sibling crate (`invoiceninja-sdk-cli`) in a
   workspace, not in this one, for three reasons:
 
   - *Dependencies.* Every entry in `Cargo.toml` is there because the library
@@ -106,7 +106,7 @@ Invoice Ninja account to contribute.
   - *Versioning.* Pre-1.0 this crate tells callers to pin an exact version
     and expect breaking changes in minor releases. A CLI's flags are a
     compatibility surface too, and one version number would force the two to
-    move in lockstep. `cargo install invoiceninjars` silently needing
+    move in lockstep. `cargo install invoiceninja-sdk` silently needing
     `--features cli` is a smaller version of the same problem.
 
   For ad-hoc probing, `examples/` already covers it and costs nothing: