docs: update PLAN, CODEBASE, README with env vars, stubs, tests, atomic state

PLAN.md:
- Add SERVER_BINARY, SERVER_PARAMS, STEAMCMD_PATH to env vars table
- Add testdata/ to project structure
- Add testing section (backend tests, stubs, frontend tests, make targets)
- Update process start flow with env var overrides

CODEBASE.md:
- Add testdata/ stubs to directory map
- Replace zustand with vitest/testing-library/jsdom in frontend deps
- Update CI to include test steps
- Update Makefile targets description
- Add atomic state machine to ProcessManager description
- Add env overrides and atomic state to design decisions
- Update data flow with env var overrides

README.md:
- Add SERVER_BINARY, SERVER_PARAMS, STEAMCMD_PATH to env vars table
- Remove FRONTEND_DIR (embedded frontend)
- Update Development section with make targets

README.md

@@ -47,8 +47,10 @@ All paths are configurable via environment variables:
 | `MODS_DIR` | `serverfiles/mods` | Local mod symlinks/copies |
 | `CFG_DIR` | `serverfiles/cfg` | Server configuration `.cfg` files |
 | `PROFILES_DIR` | `serverfiles/profiles` | Arma 3 profile and log directory |
-| `FRONTEND_DIR` | `../frontend/dist` | Built frontend assets |
 | `LISTEN` | `:8080` | HTTP listen address |
+| `SERVER_BINARY` | `arma3server_x64` | Server binary filename (overrides platform default) |
+| `SERVER_PARAMS` | `-server -world=empty ...` | Override server launch parameters |
+| `STEAMCMD_PATH` | `steamcmd` | Path to steamcmd binary |
 
 ## Automation
 
@@ -150,14 +152,23 @@ frontend/
 ## Development
 
 ```bash
-# Backend
-cd backend && go run ./cmd/server
+# Start both backend + frontend with hot reload (copies stubs to dev-deploy)
+make dev
 
-# Frontend (hot reload)
-cd frontend && npm run dev
+# Start backend only
+make run
 
-# Lint
-cd frontend && npm run lint
+# Run all tests
+make test
+
+# Run backend tests only
+make test-backend
+
+# Run frontend tests only
+make test-frontend
+
+# Build for production
+make build
 ```
 
 ## License