fix(prefill): respect loadOnScroll flag in _prefillNext to prevent auto-load in trigger mode (#886)

[Hoang130203]

Problem

Fixes #886

When using trigger mode, a page load is triggered automatically on reload — even though the user never clicked the trigger button.

Root cause

The flow on reload with trigger mode:

1. IAS initialises with loadOnScroll: true (default — user doesn't need to set loadOnScroll: false explicitly when using a trigger)
2. BINDED event fires, handlers execute in registration order:
   • Trigger.bind() → update(pageIndex) → voter returns true → disableLoadOnScroll() ✅ loadOnScroll = false
   • prefill.prefill() → _prefillNext() → checks distance() → if the browser restored scroll position near the sentinel (within negativeMargin), distance <= 0 → this.ias.next() is called unconditionally ❌

_prefillNext() called next() directly without checking loadOnScroll, bypassing the trigger mode guard entirely.

Fix

Add a loadOnScroll check at the top of _prefillNext():

_prefillNext() {
  if (!this.ias.loadOnScroll) {
    return;  // trigger mode (or explicit loadOnScroll: false) — don't auto-load
  }
  // ... rest unchanged
}

This is consistent with how the main HIT event handler in infinite-ajax-scroll.js already guards next():

this.on(Events.HIT, () => {
  if (!this.loadOnScroll) {
    return;
  }
  this.next();
});

Prefill should respect the same flag.

Repro steps

1. Configure IAS with a trigger (no explicit loadOnScroll: false)
2. Scroll down until the trigger button is visible in the viewport
3. Reload the page (browser restores scroll position)
4. Before fix: next page loads automatically
5. After fix: trigger button is shown, no auto-load occurs