avm2: Normalize flash.globalization.Collator.compare to -1/0/1

[falpi]

Summary

flash.globalization.Collator.compare() returned a raw character-code delta instead of a
normalized -1 / 0 / 1. Flash Player's Collator.compare always returns exactly the sign,
and the Flex framework's collection sorting relies on that: the mismatch drives
spark.collections.Sort.findItem()'s binary search into an infinite loop, freezing any app
that sorts a Spark DataGrid string column by clicking its header (CPU pegged, no error thrown).

Root cause

Ruffle's Collator.compare stub delegated to String.localeCompare, which returns a
character-code difference rather than a normalized result:

"Rossi".localeCompare("Bianchi") // 'R'(82) - 'B'(66) == 16, not 1

spark.collections.Sort.findItem() (the binary search used when the collection view resolves an
item's index) branches on the comparison result with an exact switch:

switch (direction) {
    case -1: upperBound = index - 1; break;
    case  0: /* found */            break;
    case  1: lowerBound = index + 1; break;
}

A delta such as 16 matches none of these cases, so neither bound is updated,
lowerBound <= upperBound stays true forever, and the comparator
(SortField.stringCompare → Collator.compare) is called endlessly.

The path reached from a header click is:

DataGrid.sortByColumns → ListCollectionView.refresh
  → Grid.dataProvider_collectionChangeHandler → updateCaretForDataProviderChange
    → ListCollectionView.getItemIndex → Sort.findItem
      → Sort.internalCompare → SortField.stringCompare → Collator.compare   // spins here

Reproduction

A minimal Spark DataGrid with inline data. Compile with the Flex 4.6 SDK and open the SWF in
Ruffle, then click the Name column header to sort. Before this patch the player hangs; after
it, the column sorts normally.

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
              xmlns:s="library://ns.adobe.com/flex/spark"
              xmlns:mx="library://ns.adobe.com/flex/mx">

    <fx:Declarations>
        <mx:ArrayCollection id="people">
            <fx:Object name="Rossi"/>
            <fx:Object name="Bianchi"/>
            <fx:Object name="Verdi"/>
        </mx:ArrayCollection>
    </fx:Declarations>

    <s:DataGrid dataProvider="{people}" width="300" height="120">
        <s:columns>
            <s:ArrayList>
                <s:GridColumn dataField="name" headerText="Name"/>
            </s:ArrayList>
        </s:columns>
    </s:DataGrid>

</s:Application>

Any string values whose comparison delta is not exactly ±1 reproduce it (i.e. almost all real
data — here 'R' - 'B' == 16).

Fix

Normalize the comparison result to its sign in Collator.compare, matching Flash Player:

var result:int = string1.localeCompare(string2);
return (result < 0) ? -1 : ((result > 0) ? 1 : 0);

Notes

• Collator.compare is still a stub for locale-aware collation (it uses code-point ordering);
  this change only corrects the shape of the return value, which is what the framework depends on.
• Custom sortCompareFunctions that already return -1/0/1 were unaffected and keep working; this
  fixes the default string-sort path that goes through the Collator.

Checklist

• I, a human, have self-reviewed this PR and fully understand the changes within.
• I have made or updated tests where possible.
• All of my commits are properly scoped, compile successfully, and pass all tests.
• This PR does not make sense to split up into smaller PRs.
• An LLM was involved in the authoring of this code.