test: add coordinate system regression demo

[joeykchen]

Summary

• add a CoordinateSystem regression demo for the unified SPX/Godot coordinate conversion
• visualize the SPX origin, axis directions, and the sprite's independent logical root
• verify costume anchors at the geometric center and two off-center asset coordinates
• explicitly declare the SVG source size as 120 × 80 so costume centers use the correct asset pixel space
• exercise rotation, LeftRight flipping, movement, pen drawing, and stamping while checking logical-position invariants

Impact

The demo provides an interactive regression fixture for the logical-root/render-node split. It makes anchor conversion and render-offset regressions visible without changing runtime behavior.

Verification

cd test/CoordinateSystem
spx runnative

• startup anchor checks: PASS
• left/right flip round trip: PASS
• related SPX coordinate and render-offset tests: PASS
• JSON and SVG validation: PASS

Verified on macOS amd64 with the native Godot SPX runtime at a9b06008a.

Related

• Refactor SPX coordinate ownership and separate logical, rendering, and physics nodes #1702
• refactor(spx): centralize coordinate conversion godot#327
• refactor: separate logical and render coordinates #1703

[fennoai]

No issues found in this review.

Checked the added CoordinateSystem demo files and validated the new JSON metadata. I could not run the native demo locally because the spx executable is not installed in this workspace.