fix: add unsubscribe mechanism to prevent callback memory leaks

[atharrva01]

Summary

Fixes a memory leak caused by push-only subscriptions in Phoenix when navigating between experiments. Route-scoped Angular components subscribed to long-lived singleton services without an unsubscribe mechanism, causing callbacks to accumulate and destroyed components to remain in memory.

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Issue Description

Core services (EventDisplay, ActiveVariable) stored callbacks in internal arrays but did not provide a way to remove them. When Angular components were destroyed on navigation, their callbacks remained registered and retained component instances via closures.

// Before
this.onEventsChange.push(callback);

Over time this led to unbounded memory growth and increasing event dispatch cost.

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Steps to Reproduce

1. Open Phoenix and DevTools → Memory
2. Take a heap snapshot
3. Navigate repeatedly between experiments (ATLAS → CMS → LHCb)
4. Take another snapshot

Before fix: callback counts and retained component instances grow with each navigation.

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Fix

Subscription APIs now return an unsubscribe function, and Angular components clean up subscriptions on destroy.

// Core API
const unsubscribe = listenToLoadedEventsChange(cb);

// Component
ngOnDestroy() {
  unsubscribe();
}

This makes subscriptions lifecycle-safe and allows destroyed components to be garbage-collected.

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Impact

• Callback counts remain stable across navigation
• Destroyed components are released
• Memory usage stays flat over long sessions
• No behavior change for existing consumers

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Backward Compatibility

Fully backward-compatible. Existing callers that ignore the returned unsubscribe function continue to work unchanged.

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[atharrva01]

Hi @EdwardMoyse 👋,
This PR fixes a memory leak caused by persistent subscriptions retaining destroyed components across navigation. Subscriptions are now lifecycle-safe and cleaned up on destroy.

👍 Would appreciate a review whenever you get time. Thanks!