卫戍协议：盟约 web remake — initial import

Online co-op browser remake of Arknights' Stronghold Protocol: Alliance
(client-simulated combat, official waves/bonds/items/strategies, 3D board,
facing wheel, operator loadout, official emotes, deployment scripts).

server/sim/projectiles.js

@@ -0,0 +1,78 @@
+// server/sim/projectiles.js — projectile flight & impact (DESIGN §5.4 addProjectile).
+//
+// A projectile homes on `target` (a unit) or flies to a fixed point `to` ({x,y}). On arrival `onHit(ctx)` runs
+// with ctx = { battle, projectile, target (null if it died / point shot), x, y }. When a homing target dies
+// mid-flight the projectile fizzles, unless `hitDead: true` (then it lands at the last known position).
+
+import { PROJECTILE_SPEED } from './constants.js';
+
+let seq = 0;
+const fin = (v, d) => (typeof v === 'number' && Number.isFinite(v) ? v : d);
+
+export class ProjectileSystem {
+  constructor(battle) {
+    this.battle = battle;
+    /** @type {object[]} */
+    this.list = [];
+  }
+
+  add(p) {
+    const from = p.from || { x: 0, y: 0 };
+    // non-finite coordinates would never "arrive" (NaN distance) and leak the projectile for maxAge
+    const fx = fin(from.x, 0), fy = fin(from.y, 0);
+    const target = p.target && typeof p.target === 'object' ? p.target : null;
+    const proj = {
+      id: ++seq,
+      x: fx,
+      y: fy,
+      target,
+      // the target "life" it was fired at: an op that dies and is redeployed mid-flight is a new target (fizzle)
+      tseq: target ? target.deploySeq : 0,
+      tx: fin(p.to ? p.to.x : (target ? target.x : fx), fx),
+      ty: fin(p.to ? p.to.y : (target ? target.y : fy), fy),
+      speed: p.speed > 0 ? p.speed : PROJECTILE_SPEED,
+      onHit: p.onHit ?? null,
+      visual: p.visual ?? 'arrow',
+      source: p.source ?? (p.from && p.from.id != null ? p.from : null),
+      hitDead: !!p.hitDead,
+      data: p.data ?? null,
+      age: 0,
+      maxAge: p.maxAge > 0 ? p.maxAge : 10,
+    };
+    this.list.push(proj);
+    return proj;
+  }
+
+  update(dt) {
+    if (!this.list.length) return;
+    const keep = [];
+    const arrived = [];
+    for (const p of this.list) {
+      p.age += dt;
+      if (p.target) {
+        if (p.target.alive && !p.target.hidden && p.target.deploySeq === p.tseq) { p.tx = fin(p.target.x, p.tx); p.ty = fin(p.target.y, p.ty); }
+        else if (!p.hitDead) continue; // fizzle
+        else p.target = null;
+      }
+      const dx = p.tx - p.x, dy = p.ty - p.y;
+      const d = Math.hypot(dx, dy);
+      const step = p.speed * dt;
+      if (d <= step || p.age >= p.maxAge) {
+        p.x = p.tx; p.y = p.ty;
+        arrived.push(p);
+      } else {
+        p.x += (dx / d) * step;
+        p.y += (dy / d) * step;
+        keep.push(p);
+      }
+    }
+    this.list = keep;
+    for (const p of arrived) {
+      if (!p.onHit) continue;
+      const t = p.target && p.target.alive && p.target.deploySeq === p.tseq ? p.target : null;
+      this.battle._safe(() => p.onHit({ battle: this.battle, projectile: p, target: t, x: p.x, y: p.y }), 'projectile.onHit', p.source);
+    }
+  }
+
+  clear() { this.list = []; }
+}