Add production build pipeline (#46)

## Summary
- split development dynamic linking from production builds
- add headless processed-asset generation
- add a separate production CI cache and static Bevy/Rust linkage verification
- keep custom debug features enabled for production builds behind a feature gate

Reviewed-on: #46
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

README.md

@@ -13,3 +13,21 @@ cargo run -- --debug-match --players 2 --local-port 7000 \
 cargo run -- --debug-match --players 2 --local-port 7001 \
   --net-player 0=127.0.0.1:7000 --net-player 1=local
 ```
+
+## Production Build
+
+Production builds use the `production` Cargo profile and do not dynamically
+link Bevy or Rust. Development builds retain Bevy dynamic linking for faster
+iteration. Cargo stores the production profile under `target/production`,
+separate from development artifacts under `target/debug`.
+
+```sh
+cargo run --locked \
+  --bin process-assets --no-default-features --features debug_features
+cargo build --locked \
+  --profile production --no-default-features --features debug_features
+```
+
+The asset preprocessing command must be run from the repository root. It
+populates `imported_assets/Default`, which is then consumed by the production
+game without running the asset processor at startup.