Split Rust game src into modules; update AGENTS.md for dual toolchain

- Refactor apps/game/src/main.rs into modules: state.rs, camera.rs, ui/, gameplay/
- Add gameplay/galaxy_creation.rs scaffold (stubs only, no new logic)
- Update root AGENTS.md: separate TS workspace vs Rust game commands
- Update apps/AGENTS.md: docs/game/site use two toolchains, not one
- Add apps/game/AGENTS.md: build commands, module layout, naming conventions, Plugin pattern

apps/game/AGENTS.md

@@ -0,0 +1,78 @@
+# apps/game — Bevy/Rust Game Client
+
+Single-player space exploration RPG built with [Bevy 0.16](https://bevyengine.org).
+Crate name: `void-nav`.
+
+This app is **independent of the pnpm/TS workspace** — no Vite, no React, no Tailwind. It builds with plain `cargo` from this directory.
+
+## Commands
+
+```bash
+cargo run               # Build + launch the game
+cargo check             # Fast typecheck (no codegen)
+cargo build             # Build without running
+cargo clippy            # Lint
+cargo test              # Run unit tests
+```
+
+## Module Layout
+
+```
+src/
+├── main.rs              # App builder: plugins, state init, system registration
+├── state.rs             # AppState enum (MainMenu, GalaxyCreation, CharacterCreation, InGame, Options)
+├── camera.rs            # Camera spawn
+├── ui/                  # UI screens (menus, HUD, etc.)
+│   ├── mod.rs
+│   └── main_menu.rs
+└── gameplay/            # Non-UI gameplay systems
+    ├── mod.rs
+    └── galaxy_creation.rs
+```
+
+### When to add a file vs a folder
+
+- **Default**: one file per feature (e.g. `main_menu.rs`, `galaxy_creation.rs`).
+- **Promote to a folder** when the file exceeds ~300 lines, mixes UI + logic + data, or needs shared private helpers.
+- A promoted folder should expose its public API via `mod.rs` and ideally bundle its systems into a Bevy `Plugin` (see pattern below).
+
+### Plugin pattern (recommended once a folder exists)
+
+```rust
+// src/gameplay/galaxy_creation/mod.rs
+pub struct GalaxyCreationPlugin;
+
+impl Plugin for GalaxyCreationPlugin {
+    fn build(&self, app: &mut App) {
+        app.add_systems(OnEnter(AppState::GalaxyCreation), setup_galaxy_creation)
+            .add_systems(OnExit(AppState::GalaxyCreation), despawn_galaxy_creation)
+            .add_systems(Update, (
+                /* update systems */
+            ).run_if(in_state(AppState::GalaxyCreation)));
+    }
+}
+```
+
+Then `main.rs` collapses to `app.add_plugins((..., GalaxyCreationPlugin))`.
+
+## Naming Conventions
+
+Follows [Rust RFC 344](https://rust-lang.github.io/api-guidelines/naming.html). Bevy layers these additional conventions:
+
+| Item | Convention | Example |
+|---|---|---|
+| Files, modules, directories | `snake_case` | `galaxy_creation.rs`, `main_menu.rs` |
+| Crate name | `snake_case` | `void-nav` |
+| Structs, enums, enum variants | `UpperCamelCase` | `AppState`, `GalaxyCreation` |
+| Components | `UpperCamelCase` (suffix optional) | `Player`, `MainMenuUi` |
+| Resources | `UpperCamelCase` | `ClearColor`, `Time` |
+| States | `UpperCamelCase` + `State` suffix | `AppState`, `GameState` |
+| Plugins | `UpperCamelCase` + `Plugin` suffix | `GalaxyCreationPlugin` |
+| Functions, systems, locals | `snake_case`, verb-first | `spawn_camera`, `setup_main_menu` |
+| Constants, statics | `SCREAMING_SNAKE_CASE` | `MAX_HEALTH` |
+
+## Architecture Notes
+
+- **State machine driven**: each `AppState` variant has its own UI/systems wired via `OnEnter` / `OnExit` / `Update` (latter guarded by `in_state`).
+- **Persistence**: SpacetimeDB will be the authoritative backend (via the Rust SDK, not the TS bindings). Not yet wired up.
+- **No `module_bindings/` here**: TS bindings live in the TS workspace and are consumed by the docs prototype.