docs: add nextest as the fast local test run

The local gate serializes tests because of a native font race. nextest
gives each test its own process, so the whole suite can run in parallel
while iterating. It is local only: it skips doctests and the isolated
render tests, so the gate stays as it is. Add it to the Nix shell.

AGENTS.md

@@ -54,6 +54,7 @@
 - Dependency changes must satisfy `deny.toml`. A new license or duplicate version needs an entry there with its reason.
 
 ## Validation
+- For a fast full-suite run while iterating, use `cargo nextest run --workspace --all-features`. Each test gets its own process, so it runs in parallel despite the native-font race that keeps the gate serial. It skips doctests and the isolated render tests, so it does not replace the gate.
 - Full local CI is `./tools/lint-and-test.sh`. It needs the .NET SDK selected by `global.json`. Without it, `cargo test` still runs the Rust source guards in `tests/repository_guards` (process sites, config writers, shared dependencies, use statements, no Python), but not the C# checks, including the source-coverage check that finds `.rs` files Cargo never compiles.
 - That script runs the C# repository checks (assets, version, nixpkgs recipe, source coverage, legacy tools), the C# tool tests (which also run the packaging shell contracts), `cargo fmt --all -- --check`, clippy with all targets/features, all-feature tests, and no-default-feature tests.
 - Run `git diff --check` before handoff. When relevant files are untracked and the index must stay unchanged, check those files directly as well.

CONTRIBUTING.md

@@ -125,6 +125,16 @@ cargo test session::
 cargo test -p wayscriber-configurator
 ```
 
+For a fast run of the whole suite, use [cargo-nextest](https://nexte.st/), which the Nix shell
+includes. It gives each test its own process, so it can run in parallel despite the native font
+race that keeps the local CI entry point serial:
+
+```bash
+cargo nextest run --workspace --all-features
+```
+
+It skips doctests and the isolated render tests, so it does not replace that entry point.
+
 Before submitting a broad or cross-package change, run the local CI entry point:
 
 ```bash

flake.nix

@@ -33,6 +33,7 @@
         developmentInputs = with pkgs; [
           rustToolchain
           cargo-deny
+          cargo-nextest
           util-linux
           pkg-config
           cairo