feat(sound): wire sound hooks into game engine

Sound integration points:
- writeCode() -> playClick()
- buyBuilding() -> playBuild()
- buyProject() -> playProject()
- checkMilestones() -> playMilestone()
- showPhaseTransition() -> playFanfare() + updateAmbientPhase()
- renderDriftEnding() -> playDriftEnding()
- renderBeaconEnding() -> playBeaconEnding()
- Game init -> startAmbient() on first user interaction
- toggleMute() -> onMuteChanged() for ambient gain control

All hooks use typeof guard to avoid errors if sound.js fails to load.

js/engine.js

@@ -283,6 +283,7 @@ function checkMilestones() {
                 G.milestones.push(m.flag);
                 log(m.msg, true);
                 showToast(m.msg, 'milestone', 5000);
+                if (typeof Sound !== 'undefined') Sound.playMilestone();
 
                 // Check phase advancement
                 if (m.at) {
@@ -296,6 +297,10 @@ function checkMilestones() {
                             if (pNum > _shownPhaseTransition) {
                                 _shownPhaseTransition = pNum;
                                 showPhaseTransition(pNum);
+                                if (typeof Sound !== 'undefined') {
+                                    Sound.playFanfare();
+                                    Sound.updateAmbientPhase(pNum);
+                                }
                             }
                         }
                     }
@@ -356,6 +361,7 @@ function buyBuilding(id) {
     log(`Built ${def.name} ${label} (total: ${totalBuilt})`);
     // Particle burst on purchase
     const btn = document.querySelector('[onclick="buyBuilding(\'' + id + '\')"]');
+    if (typeof Sound !== 'undefined') Sound.playBuild();
     if (btn) {
         const rect = btn.getBoundingClientRect();
         const cx = rect.left + rect.width / 2;
@@ -396,6 +402,7 @@ function buyProject(id) {
 
     updateRates();
     // Gold particle burst on project completion
+    if (typeof Sound !== 'undefined') Sound.playProject();
     const pBtn = document.querySelector('[onclick="buyProject(\'' + id + '\')"]');
     if (pBtn) {
         const rect = pBtn.getBoundingClientRect();
@@ -437,6 +444,7 @@ function renderDriftEnding() {
     // Fade-in animation
     el.classList.add('fade-in');
     el.classList.add('active');
+    if (typeof Sound !== 'undefined') Sound.playDriftEnding();
 
     // Log the ending text with delays for dramatic effect
     const lines = [
@@ -479,8 +487,7 @@ function renderBeaconEnding() {
         </button>
     `;
     document.body.appendChild(overlay);
-
-    // Create particle/light ray container
+    if (typeof Sound !== 'undefined') Sound.playBeaconEnding();
     const particleContainer = document.createElement('div');
     particleContainer.id = 'beacon-ending-particles';
     document.body.appendChild(particleContainer);
@@ -762,6 +769,7 @@ function writeCode() {
     }
     // Float a number at the click position
     showClickNumber(amount, comboMult);
+    if (typeof Sound !== 'undefined') Sound.playClick();
     updateRates();
     checkMilestones();
     render();