test: add coordinate system regression demo

test/CoordinateSystem/README.md

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+# SPX coordinate-system regression demo
+
+This demo exercises the coordinate ownership introduced by:
+
+- goplus/spx#1702
+- goplus/godot#327
+- goplus/spx#1703
+
+Expected behavior:
+
+1. The stage center is SPX `(0, 0)`, `+X` points right, and `+Y` points up.
+2. The black cross is the sprite's logical root. Pressing `Space` cycles the
+   same SVG through three costume centers. The selected colored anchor must
+   stay on the black cross while the rendered SVG moves around it.
+   The SVG source dimensions are declared explicitly as `120 × 80`, so each
+   center is interpreted in the same source-image pixel coordinate space.
+3. `R` rotates around the logical root. `F` flips left/right around the same
+   root. `N` restores normal rotation.
+4. `P` draws a cyan pen segment in SPX `+Y`, so the segment must go upward.
+5. `S` stamps the current rendered costume and then moves the live sprite to
+   the right. The stamp must remain exactly where the rendered costume was.
+6. Arrow keys move both the logical root and its rendering together. `C`
+   resets the demo and `E` clears pen drawings and stamps.
+
+Run with the native SPX runtime:
+
+```sh
+cd test/CoordinateSystem
+spx runnative
+```

test/CoordinateSystem/RootMarker.spx

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+// The marker is a separate sprite at Subject's logical SPX position. It must
+// not inherit Subject's costume render offset, rotation, or horizontal flip.

test/CoordinateSystem/Subject.spx

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+import "fmt"
+import "math"
+
+const coordinateStep = 40.0
+
+var (
+	expectedX = 0.0
+	expectedY = 100.0
+	flipped   bool
+)
+
+func syncLogicalRoot() {
+	RootMarker.SetXYpos xpos, ypos
+}
+
+func acceptLogicalPosition() {
+	// Motion can be clamped by the stage fence. Use the resulting logical SPX
+	// position as the baseline for later costume/rotation invariants.
+	expectedX, expectedY = xpos, ypos
+}
+
+func verifyLogicalPosition(action string) {
+	if math.Abs(xpos-expectedX) > 0.001 || math.Abs(ypos-expectedY) > 0.001 {
+		msg := fmt.Sprintf("FAIL %s: got (%.1f, %.1f), want (%.1f, %.1f)", action, xpos, ypos, expectedX, expectedY)
+		println msg
+		say msg
+		return
+	}
+	println fmt.Sprintf("PASS %s: logical SPX position = (%.1f, %.1f)", action, xpos, ypos)
+}
+
+func reportState(action string) {
+	verifyLogicalPosition action
+	syncLogicalRoot
+	say fmt.Sprintf("%s\nanchor=%s\nSPX=(%.0f, %.0f)", action, costumeName, xpos, ypos)
+}
+
+func resetCoordinateDemo() {
+	expectedX, expectedY = 0, 100
+	setXYpos expectedX, expectedY
+	setRotationStyle Normal
+	setHeading 90
+	setCostume 0
+	flipped = false
+	reportState "reset"
+}
+
+onStart => {
+	setPenSize 4
+	setPenColor HSB(52, 80, 90)
+	resetCoordinateDemo
+
+	// Exercise each asset-space center once. Only RenderRoot should move.
+	for i := 0; i < 3; i++ {
+		setCostume i
+		reportState "startup center conversion"
+		wait 0.8s
+	}
+	setCostume 0
+	reportState "ready"
+}
+
+onClick => {
+	setCostume Next
+	reportState "click: next costume center"
+}
+
+onKey KeySpace, => {
+	setCostume Next
+	reportState "next costume center"
+}
+
+onKey KeyUp, => {
+	changeYpos coordinateStep
+	acceptLogicalPosition
+	reportState "move +Y"
+}
+
+onKey KeyDown, => {
+	changeYpos -coordinateStep
+	acceptLogicalPosition
+	reportState "move -Y"
+}
+
+onKey KeyLeft, => {
+	changeXpos -coordinateStep
+	acceptLogicalPosition
+	reportState "move -X"
+}
+
+onKey KeyRight, => {
+	changeXpos coordinateStep
+	acceptLogicalPosition
+	reportState "move +X"
+}
+
+onKey KeyR, => {
+	setRotationStyle Normal
+	turn 30
+	reportState "rotate around logical root"
+}
+
+onKey KeyF, => {
+	setRotationStyle LeftRight
+	if flipped {
+		setHeading 90
+	} else {
+		setHeading -90
+	}
+	flipped = !flipped
+	reportState "flip around logical root"
+}
+
+onKey KeyN, => {
+	setRotationStyle Normal
+	setHeading 90
+	flipped = false
+	reportState "normal rotation"
+}
+
+onKey KeyP, => {
+	penDown
+	changeYpos 80
+	penUp
+	acceptLogicalPosition
+	reportState "pen moved +Y"
+}
+
+onKey KeyS, => {
+	stamp
+	changeXpos 140
+	acceptLogicalPosition
+	reportState "stamp then move +X"
+}
+
+onKey KeyC, => {
+	resetCoordinateDemo
+}
+
+onKey KeyE, => {
+	eraseAll
+	reportState "erase pen and stamps"
+}

test/CoordinateSystem/assets/index.json

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+{
+  "run": {
+    "title": "SPX Coordinate System",
+    "width": 640,
+    "height": 480
+  },
+  "backdrops": [
+    {
+      "name": "coordinate-grid",
+      "path": "coordinate-grid.svg"
+    }
+  ],
+  "backdropIndex": 0,
+  "zorder": [
+    {
+      "type": "sprite",
+      "target": "Subject",
+      "x": 0,
+      "y": 100
+    },
+    {
+      "type": "sprite",
+      "target": "RootMarker",
+      "x": 0,
+      "y": 100
+    }
+  ]
+}

test/CoordinateSystem/assets/sprites/RootMarker/index.json

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+{
+  "costumes": [
+    {
+      "bitmapResolution": 1,
+      "imageWidth": 24,
+      "imageHeight": 24,
+      "name": "logical-root",
+      "path": "root-marker.svg",
+      "x": 12,
+      "y": 12
+    }
+  ],
+  "costumeIndex": 0,
+  "heading": 90,
+  "isDraggable": false,
+  "rotationStyle": "none",
+  "size": 1,
+  "visible": true,
+  "x": 0,
+  "y": 100
+}

test/CoordinateSystem/assets/sprites/Subject/index.json

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+{
+  "costumes": [
+    {
+      "bitmapResolution": 1,
+      "imageWidth": 120,
+      "imageHeight": 80,
+      "name": "geometric-center",
+      "path": "subject.svg",
+      "faceRight": 0,
+      "x": 60,
+      "y": 40
+    },
+    {
+      "bitmapResolution": 1,
+      "imageWidth": 120,
+      "imageHeight": 80,
+      "name": "asset-20-20",
+      "path": "subject.svg",
+      "faceRight": 0,
+      "x": 20,
+      "y": 20
+    },
+    {
+      "bitmapResolution": 1,
+      "imageWidth": 120,
+      "imageHeight": 80,
+      "name": "asset-100-60",
+      "path": "subject.svg",
+      "faceRight": 0,
+      "x": 100,
+      "y": 60
+    }
+  ],
+  "costumeIndex": 0,
+  "heading": 90,
+  "isDraggable": false,
+  "rotationStyle": "normal",
+  "size": 1,
+  "visible": true,
+  "x": 0,
+  "y": 100
+}

test/CoordinateSystem/main.spx

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+onStart => {
+	println "CoordinateSystem demo ready"
+	println "Space: costume center | arrows: move | R/F/N: rotate/flip/reset | P: +Y pen | S: stamp | C: reset | E: erase"
+}