UAT-8a: rotate the rect, not a separate axis vector

reduceAxisState 'rotate' now changes state.rect.angle and re-derives
state.axis via axisFromRect(newRect). The rectangle rotates as a whole;
the axis for Measure is derived from the rotated rect's long side.

2 new regression tests: rect.angle changes on rotate, axis is
re-derived from rotated rect. 109/109 tests green.

frontend/src/lib/core/axis.js

@@ -274,8 +274,10 @@ export function reduceAxisState(state, action) {
 		case 'rotate': {
 			const angle = action.angle;
 			if (angle === 0) return state;
-			const newAxis = rotateAxis(state.axis, angle);
-			return { ...state, axis: newAxis, approved: false };
+			// Rotate the RECTANGLE, not a separate axis vector. The rect's angle
+			// changes, and the axis is re-derived from the rotated rect.
+			const newRect = { ...state.rect, angle: state.rect.angle + angle };
+			return { ...state, rect: newRect, axis: axisFromRect(newRect), approved: false };
 		}
 		case 'approve': {
 			return { ...state, approved: true };

frontend/tests/unit/axis.test.js

@@ -319,6 +319,25 @@ describe('reduceAxisState', () => {
 		expect(s.axis.y).toBeCloseTo(1, 6);
 	});
 
+	it('rotate changes the rect angle (not just a separate axis vector)', () => {
+		let s = createAxisState(rect);
+		const originalAngle = s.rect.angle;
+		s = reduceAxisState(s, { type: 'rotate', angle: Math.PI / 4 });
+		// The rect itself must have rotated
+		expect(s.rect.angle).toBeCloseTo(originalAngle + Math.PI / 4, 6);
+		// And the axis is re-derived from the rotated rect
+		expect(s.axis).toEqual(axisFromRect(s.rect));
+	});
+
+	it('rotate by 90 deg: rect angle changes, axis is perpendicular to original', () => {
+		let s = createAxisState(rect);
+		s = reduceAxisState(s, { type: 'rotate', angle: Math.PI / 2 });
+		expect(s.rect.angle).toBeCloseTo(Math.PI / 2, 6);
+		// Original axis was +x (1,0). After 90 deg rect rotation, axis is +y (0,1)
+		expect(s.axis.x).toBeCloseTo(0, 6);
+		expect(s.axis.y).toBeCloseTo(1, 6);
+	});
+
 	it('rotate composes (rotates from current axis)', () => {
 		let s = createAxisState(rect);
 		s = reduceAxisState(s, { type: 'rotate', angle: Math.PI / 2 });