Explore collection-initializer API (Option A') as alternative to fluent modifiers

@codemonkeychris is already working on this.

Since Apr 17, 2026.

Spec: [docs/spec/019-collection-initializer-api-exploration.md](https://github.com/microsoft/microsoft-ui-reactor/blob/main/docs/spec/019-collection-initializer-api-exploration.md)

Today's fluent API mixes three syntaxes — `.Method()` chains, `with { }` for record props, and `.Set(x => ...)` as an escape hatch. Spec 019 explores **Option A'**: `new` + object initializers + C# 12 collection expressions (`Children = [..]`). Same result, properties grouped, no `.Set()` escape hatch, spread works natively for dynamic children.

### Example — TodoApp header

**Today (fluent):**

```csharp
Text("todos").FontSize(36)
.Set(t => t.FontWeight = FontWeights.Light)
.Foreground(AccentText)
.HAlign(HorizontalAlignment.Center)
.Margin(0, 16, 0, 8)
```

**Option A' (`new` + initializer):**

```csharp
new Text("todos") {
FontSize = 36,
Weight = FontWeights.Light,
Foreground = AccentText,
HAlign = HorizontalAlignment.Center,
Margin = Thick(0, 16, 0, 8),
}
```

The `FontWeight` no longer needs `.Set()`, all properties sit in one `{ }` block, and dynamic child lists become `Children = [..items.Select(Render)]` instead of `.ToArray()` into `params`.

See the spec for the full analysis across TodoApp, Outlook MessageRow, charts, and the comparison matrix vs. Options A/B/C/E.