feat: add take-screenshots flake app for headless capture

Expose a `nix run .#take-screenshots` app that runs a binary inside an
Xvfb display with lavapipe software Vulkan and captures PNG snapshots
via ImageMagick. Useful for smoke-testing the Bevy renderer in
environments without a GPU (CI, sandboxed shells, agents).

Usage:
  nix run .#take-screenshots -- EXE NUM DELAY_START PAUSE_INBETWEEN [OUTPUT_DIR]

The script picks the first free :N >= 99, locates the lavapipe ICD via
pkgs.mesa with a fallback to /run/opengl-driver (NixOS), reuses the
dev shell's runtimeLibs in LD_LIBRARY_PATH, and traps EXIT for cleanup.

README updated with usage and a worked example.

README.md

@@ -34,3 +34,28 @@ nix develop --command bash -c "cargo build"
 2.  Clone the repository.
 3.  Navigate to the project directory.
 4.  Run the game using the command: `nix develop --command bash -c "cargo run"`
+
+## Headless Screenshots
+
+The flake exposes a `take-screenshots` app that launches a binary inside an
+Xvfb display backed by lavapipe (software Vulkan), waits, and captures one or
+more PNG screenshots. Useful for smoke-testing rendering without a real GPU.
+
+```
+nix run .#take-screenshots -- EXE NUM DELAY_START PAUSE_INBETWEEN [OUTPUT_DIR]
+```
+
+*   `EXE` — path to the executable to launch
+*   `NUM` — number of screenshots to take
+*   `DELAY_START` — seconds to wait after launch before the first shot
+*   `PAUSE_INBETWEEN` — seconds between consecutive shots
+*   `OUTPUT_DIR` — where to write `shot-NNN.png` files (default: current directory)
+
+Example, capturing three frames of the game one second apart after a six-second
+warm-up (Bevy + software Vulkan needs roughly that long to render its first
+frame):
+
+```
+cargo build
+nix run .#take-screenshots -- ./target/debug/bglga 3 6 1 ./shots
+```