build(ground-suite): add a separate pinned Flutter shell

Provide a separate Flutter development shell from the existing locked
Nix input and document the application build commands.

Validation: locked environment/version probe; no native application
qualification is claimed.

docs/src/development.md

@@ -52,3 +52,21 @@ The strict gate requires 100% line and branch coverage for `kaptan-math`,
 core package's coverage artifacts, unions LCOV source-line records, and uses
 full LLVM source spans to merge generic branch instantiations without lowering
 the gate.
+
+## Ground suite builds
+
+The separate locked shell supplies Flutter, Dart and desktop dependencies:
+
+```sh
+nix develop .#ground-suite
+cd ground_suite
+flutter pub get --enforce-lockfile
+dart format --output=none --set-exit-if-changed lib
+dart analyze
+flutter analyze --no-pub
+flutter build web --release --no-pub
+flutter build linux --release --no-pub
+```
+
+The app currently has no widget-test suite. Windows and macOS need their native
+build and runtime checks.