Add per-app cascade configuration and auto app ID detection

[Copilot]

Cascade configuration was global. App registration required manual appId passing. Iframes with identical titles couldn't share cascade state.

Changes

Per-app cascade settings

• App definitions now accept cascadeOffset and maxCascade (defaults: 15px, 4 stack)
• registerActiveApp() stores settings in active instance
• addWindow() reads per-app settings instead of globals

Auto appId detection

• addWindow() extracts title as appId when not provided
• Eliminates explicit appId parameter in most cases

Iframe cascade defaults

• simpleIframe() enables cascade by default
• addIframeWindow() helper reduces duplication across 3 call sites

Example

// App definition with custom cascade
APP_REGISTRY.appDefinitions.utilities.myApp = {
    cascadeOffset: 20,  // Per-app override
    maxCascade: 6
};

// Before: manual appId required
addWindow(iframe, 0, 0, 0, 0, true, true, 'pixelwind', appDef);

// After: auto-detected from title
addIframeWindow(simpleIframe('https://example.com', { title: 'pixelwind' }));

Multiple iframes with title "Boids Test" now share cascade state automatically:

Windows cascade with 15px offset, tracked in single APP_REGISTRY.activeInstances['Boids Test'] entry.

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