Implements visual tests for breakpoint hover states in the editor gutter
with three states:
1. **No hover** - just line numbers
2. **Hover with indicator** - blue circle appears next to the line
3. **Hover with tooltip** - shows 'Set breakpoint' with F9 keybinding

- Added `VisualTestPlatform` that combines real Mac rendering with
controllable `TestDispatcher` for deterministic task scheduling and time
control
- Added `VisualTestAppContext::with_asset_source()` to enable SVG icon
rendering in visual tests (uses real Assets instead of empty source)
- Added F9 keybinding for ToggleBreakpoint so tooltip shows the
keybinding
- Removed debug `eprintln!` statements from editor, element, svg, and
window

The `VisualTestPlatform` enables `advance_clock()` which is essential
for testing time-based behaviors like tooltip delays. Without
deterministic time control, tests for hover states and tooltips would be
flaky.

Release Notes:

- N/A