[Feature] Add UntoldViewOptions for runtime view settings in SwiftUI

[miogds]

Implements the UntoldViewOptions split we discussed: UntoldRendererConfig stays immutable/create-time, and runtime view-host settings (target FPS being the motivating case) get their own small Equatable struct that is diff-applied to the live MTKView — no renderer recreation, no Observable conformance on the renderer. The full design rationale is included in this PR at docs/proposals/UntoldViewOptions.md.

What's in here

UntoldViewOptions — Equatable, Sendable struct with preferredFramesPerSecond, isPaused, clearColor. Usable two ways:

UntoldView(renderer: renderer, options: UntoldViewOptions(preferredFramesPerSecond: 30)) { ... }

// or modifier style (same copy-on-write pattern as .onUpdate):
UntoldView(renderer: renderer) { ... }
    .preferredFramesPerSecond(fps)
    .paused(inMenu)

When a bound SwiftUI value changes, the body re-evaluates, updateNSView/updateUIView runs, and the coordinator applies only the properties that differ from the last-applied options. Unrelated re-evaluations cost one Equatable compare and never touch the view.

Stable renderer ownership — the "renderer lives in a stable reference holder" part: SceneView's Coordinator now owns the renderer. Previously both UntoldView.init and SceneView.init ran renderer ?? UntoldRenderer.create() on every SwiftUI re-evaluation of the view struct. Now the injected renderer is adopted (or a fallback created) exactly once, on first makeNSView/makeUIView, and never swapped.

Scene content built once — UntoldView stores the @SceneBuilder closure and runs it a single time when the platform view is created, after the renderer exists (so setEntityMeshAsync has a Metal device). Previously the builder re-ran on every re-evaluation — in the SceneBuilder demo that meant new entities and mesh reloads on every timer tick.

SceneView.onInit semantic change (intentional, please review) — the block used to execute immediately at body-evaluation time, on every re-evaluation, and required the renderer to already exist. It now runs exactly once, when the platform view is created and the renderer is ready. The block is also @MainActor now. This makes SceneView().onInit { ... } with a nil renderer safe.

Scope rules going forward

Every property in UntoldViewOptions must be applicable to the live MTKView. Pipeline-affecting settings stay in UntoldRendererConfig; engine tunables (AA, post-FX, LOD, ...) stay in the engine settings API — no duplication, one source of truth each.

Not included (follow-ups per the proposal)

• renderScale (needs care: create() pins contentsScale = 1.0)
• drawsOnDemand (enableSetNeedsDisplay mode)

Testing

• swift build clean for the engine, all demos, and Sandbox — all existing SceneView(renderer:) / UntoldView(renderer:) call sites compile unchanged (new parameters are defaulted).
• New UntoldViewOptionsTests (7 tests, no Metal device needed): struct semantics, modifier copy-on-write, first-apply sets everything, diff-apply leaves unchanged properties untouched, equal options are a no-op. All pass, plus existing SceneBuilderNodeTests.