avm2: Implement flash.ui.Mouse.cursor

[falpi]

Summary

flash.ui.Mouse was missing the cursor property entirely, so assignments to
Mouse.cursor had no effect. The Text Layout Framework sets
Mouse.cursor = MouseCursor.IBEAM when the pointer moves over editable text, so
Spark TextInput / TextArea (and any TLF-based content) never displayed the
I-beam cursor — the pointer stayed an arrow.

This implements flash.ui.Mouse.cursor (getter and setter), letting content
force a specific cursor and matching Flash Player.

Root cause

flash.ui.Mouse only exposed hide/show/supportsCursor/registerCursor.
There was no cursor property, so:

• Mouse.cursor = MouseCursor.IBEAM (used by flashx.textLayout.edit.SelectionManager.setMouseCursor) was a no-op / illegal write, and
• reading Mouse.cursor was unsupported.

Ruffle already picks a cursor automatically from the object under the pointer
(arrow, button, I-beam over a TextField, …), but nothing let ActionScript
force one.

Reproduction

Minimal AS3: a grey rectangle that requests the I-beam while hovered. Without
this patch the cursor stays an arrow (and the assignment does not take effect);
with it, the pointer becomes an I-beam over the rectangle.

import flash.display.Sprite;
import flash.events.MouseEvent;
import flash.ui.Mouse;
import flash.ui.MouseCursor;

var box:Sprite = new Sprite();
box.graphics.beginFill(0xCCCCCC);
box.graphics.drawRect(0, 0, 200, 100);
addChild(box);

box.addEventListener(MouseEvent.MOUSE_OVER, function(e:MouseEvent):void {
    Mouse.cursor = MouseCursor.IBEAM;
});
box.addEventListener(MouseEvent.MOUSE_OUT, function(e:MouseEvent):void {
    Mouse.cursor = MouseCursor.AUTO;
});

The same path is exercised implicitly by any Flex Spark editable text control.

Fix

• Add a forced_cursor: Option<MouseCursor> field to the persistent MouseData.
• The native Mouse.cursor setter stores it; the getter reads it back.
  MouseCursor.AUTO maps to None — the automatic per-object cursor is kept —
  and any other value overrides that choice. Invalid strings throw
  ArgumentError avm1: Color.setTransform/setRGB sets transformed-by-script flag (fix #1906) #2008, as in Flash Player.
• In the player's cursor update, a non-None forced_cursor replaces the
  automatically-computed cursor before it is applied.

The AS3 constant ↔ internal enum mapping: AUTO→None, ARROW→Arrow,
BUTTON→Hand, IBEAM→IBeam, HAND→Grab.

Notes

• The forced cursor takes effect on the next mouse activity (the cursor is
  recomputed while processing pointer events); TLF sets it during
  rollOver/mouseMove, so it applies immediately in practice.
• Automatic cursor selection (e.g. the hand over buttons) is unchanged when
  Mouse.cursor is left at its default "auto".

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.