Add unit testing infrastructure

[dsrw]

Summary

• Add three types of tests: unit tests, VM script tests, and in-world tests
• Add nimble test command that runs all test types with proper exit codes
• Integrate tests into CI workflows (macOS and Windows get world tests, Linux skips due to server build)

Changes

Test Infrastructure

• Unit tests (tests/unit/) - test ScriptCtx, serializers without Godot
• VM tests (tests/vm/) - test vmlib API via command-line script execution
• In-world tests (vmlib/worlds/tests/) - tests that run inside Enu

Command Line Flags

• --level-dir <path> - load specific level directory
• --enu-test - enable test mode (auto-exit when scripts finish)

New API

• has_block_at(position) - check if block exists at world position
• block_color_at(position) - get color of block at world position

Bug Fixes

• Fixed voxel lookup to also check batched voxels during script execution

Known Bugs (documented with commented-out tests)

• left/right/up/down in move mode all move in forward direction

Test plan

• [x] nimble unit_tests passes
• [x] nimble vm_tests passes
• [x] nimble world_tests passes
• [x] nimble test runs all three and reports correctly