Set up gdUnit4 unit testing framework

[aebrer]

Deep Yellow currently has no automated unit tests. Integrating gdUnit4 will provide a standard Godot testing framework, enable regression catching, and make it safer to refactor core systems like the grid, action layer, and state machine.

Current Behavior

• No test directory or test runner exists.
• Core logic (grid coordinate conversion, action validation, state transitions) is only verified through manual gameplay.
• No CI step validates that the project loads without script errors beyond basic --headless --quit.

Proposed Behavior

• Add gdUnit4 as an addon (via addons/gdUnit4/) or git submodule.
• Establish a tests/ directory with a clear structure mirroring scripts/.
• Create initial test suites for the most testable, high-value systems.
• Document how to run tests locally (CLI + Godot editor).

Acceptance Criteria

• gdUnit4 is integrated and loads without errors in the project.
• tests/ directory exists with a structure matching scripts/ (e.g., tests/actions/, tests/player/, tests/procedural/).
• At least one passing test suite exists for grid_3d.gd (e.g., grid_to_world / world_to_grid round-trip, is_floor_tile / is_wall_tile logic).
• At least one passing test suite exists for the action layer (e.g., MovementAction.can_execute returns false for walls, WaitAction costs 1 turn).
• At least one passing test suite exists for the input state machine (e.g., valid state transitions, unknown state names are handled gracefully).
• README.md or docs/ is updated with instructions on running tests via the Godot editor and headless CLI.
• A headless test run command is verified to work (e.g., godot --headless -s addons/gdUnit4/bin/GdUnitCmdTool.gd or equivalent).

Context

• gdUnit4 repository
• The project uses Godot 4.6 with a custom Tracy-enabled build — verify gdUnit4 compatibility with 4.6.

Technical Notes

• gdUnit4 can be installed via the Asset Library, git submodule, or manual download into addons/. Prefer the method that's easiest to reproduce across dev machines and CI.
• grid_3d.gd has pure functions like grid_to_world, world_to_grid, is_floor_tile, and is_wall_tile that are ideal first test targets.
• The action layer (scripts/actions/action.gd and subclasses) uses the Command Pattern with can_execute() and execute() — these can be tested with mock player objects.
• input_state_machine.gd manages state registration and transitions — tests should verify signal connections and invalid-state handling.
• Avoid testing visual/renderering logic or animation timing in unit tests; focus on deterministic game logic.