UX improvements and final touches

[ripventura]

The goal of this PR is to bring final touches to the MVP, with a major focus on user experience.

Changes

• Added custom lightning when viewing the asset on immersive mode for better UX
• Improved the control of view and view models lifecycle by leveraging the Equatable SPM library
• Improved the handling of volumetric and immersive windows when transitioning between experiences
• Improved debug logging throughout the app

[claude]

Code Review

Three issues found:

1. Overlapping ornaments at the same anchor — AssetDetailView.swift

Two .ornament(attachmentAnchor: .scene(.top)) modifiers are applied to the same view (lines 46–59). The first (pre-existing) shows Text(asset.title) when hasLoadedModel == true; the PR adds a second at the same anchor showing the loading indicator when isLoadingExperience == true. These conditions are not mutually exclusive — when the model has loaded but the immersive space is still opening (phase == .opening), both ornaments render at the same .top anchor, overlapping each other. Merge them into a single .ornament(attachmentAnchor: .scene(.top)) with internal conditional logic.

2. Broken test — AssetDisplayViewModel.swift

The PR removed logger.warning("startLoading() called while already loading — ignoring") from startLoading() (replaced with a silent guard !isLoading else { return } at line 56), but the existing test duplicateStartLoadingIsIgnored in AssetDisplayViewModelTests.swift asserts that exactly one warning-level entry containing "already loading" is logged. This test will always fail.

3. AsyncThrowingStream continuation never finished on deallocation — AssetDownloadOperator.swift

The PR introduces [weak self] on the Task inside AsyncThrowingStream (lines 75–76). If self is deallocated before the task body runs, guard let self else { return } exits without ever calling continuation.finish() or continuation.finish(throwing:). Any caller for try await-ing over the stream will hang indefinitely. Either capture self strongly (the prior behavior), or add continuation.finish(throwing: CancellationError()) in the guard failure path.