Refactor pureCalculateAnchoredPosition to eliminate DOM dependency

[Copilot]

The pureCalculateAnchoredPosition function claimed to be pure with no DOM dependencies, but was directly accessing window.innerWidth and window.innerHeight when displayInVisibleViewport was enabled.

Changes:

• Add visibleViewportSize: Size parameter to pureCalculateAnchoredPosition
• Move window.innerWidth/window.innerHeight reads to caller (getAnchoredPosition)
• Pass viewport dimensions as argument instead of reading from global

// Before: reads from window global
function pureCalculateAnchoredPosition(..., settings: PositionSettings) {
  if (displayInVisibleViewport) {
    effectiveViewportRect = {
      width: window.innerWidth,
      height: window.innerHeight,
    }
  }
}

// After: accepts viewport as parameter
function pureCalculateAnchoredPosition(..., settings: PositionSettings, visibleViewportSize: Size) {
  if (displayInVisibleViewport) {
    effectiveViewportRect = {
      width: visibleViewportSize.width,
      height: visibleViewportSize.height,
    }
  }
}

The function is now truly pure and DOM-free as documented.

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