fix(frontend): lock multi-stone parallel trajectory animation

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

frontend/src/game-model.test.ts

@@ -0,0 +1,104 @@
+import { describe, expect, it } from 'vitest'
+import { GameModel } from './game-model'
+import type { ServerStoneTrajectory, StoneState } from './protocol'
+
+function stone(partial: Partial<StoneState> & Pick<StoneState, 'id' | 'team'>): StoneState {
+  return {
+    x: 0,
+    y: 30,
+    rotation: 0,
+    active: true,
+    ...partial,
+  }
+}
+
+describe('GameModel multi-path trajectory animation', () => {
+  it('returns two DrawableStones in parallel mid-trajectory', () => {
+    const model = new GameModel()
+    model.state.stones = [stone({ id: 1, team: 'red' }), stone({ id: 2, team: 'yellow' })]
+    model.state.turnTeam = 'red'
+
+    const paths: ServerStoneTrajectory[] = [
+      {
+        stone_id: 1,
+        path: [
+          [0, 10, 0],
+          [0, 11, 0.5],
+          [0, 12, 1.0],
+        ],
+      },
+      {
+        stone_id: 2,
+        path: [
+          [1, 10, 0],
+          [1, 11, 0.5],
+          [1, 12, 1.0],
+        ],
+      },
+    ]
+
+    model.startTrajectory(paths)
+    expect(model.state.animating).toBe(true)
+
+    const mid = performance.now() + 500
+    const drawn = model.tick(mid)
+
+    expect(drawn).toHaveLength(2)
+    expect(drawn.map((d) => d.team).sort()).toEqual(['red', 'yellow'])
+    // Mid-sample y ≈ 11 for both paths (y never reaches hog line → no trim shift)
+    for (const d of drawn) {
+      expect(d.y).toBeCloseTo(11, 0)
+    }
+  })
+
+  it('clears animating when elapsed reaches maxTotal on a short path', () => {
+    const model = new GameModel()
+    model.state.stones = [stone({ id: 1, team: 'red' })]
+    model.state.turnTeam = 'red'
+
+    model.startTrajectory([
+      {
+        stone_id: 1,
+        path: [
+          [0, 10, 0],
+          [0, 10.5, 0.2],
+          [0, 11, 0.4],
+        ],
+      },
+    ])
+    expect(model.state.animating).toBe(true)
+
+    const afterEnd = performance.now() + 500
+    const drawn = model.tick(afterEnd)
+
+    expect(drawn).toEqual([])
+    expect(model.state.animating).toBe(false)
+  })
+
+  it('trims thrown stone path to hog line when id is not in existing stones', () => {
+    const model = new GameModel()
+    // Only stone 1 is already on the sheet; stone 2 is the newly thrown rock.
+    model.state.stones = [stone({ id: 1, team: 'yellow', x: 0.5, y: 35 })]
+    model.state.turnTeam = 'red'
+
+    const thrownPath: [number, number, number][] = [
+      [0, 2, 0],
+      [0, 20, 1],
+      [0, 21.5, 2],
+      [0, 30, 3],
+    ]
+
+    model.startTrajectory([{ stone_id: 2, path: thrownPath }])
+    expect(model.state.animating).toBe(true)
+
+    // Immediately after start: hog-trimmed path begins at y=20 (sample before hog), t=0
+    const atStart = performance.now()
+    const drawn = model.tick(atStart)
+
+    expect(drawn).toHaveLength(1)
+    expect(drawn[0].team).toBe('red') // turnTeam fallback for unknown id
+    expect(drawn[0].y).toBeCloseTo(20, 0)
+    // Must not still be at the hack (y=2)
+    expect(drawn[0].y).toBeGreaterThan(15)
+  })
+})