Improve unsupported DisplayList content error diagnostics

[Kyle-Ye]

Summary

• report the item identity and unsupported content head when conversion fails
• suggest the supported kebab-case spelling for known camel-case variants
• add regression coverage for platformView using DemoPlatformViewFactory

Why

DisplayList descriptions containing platformView previously failed with a generic content-seed error that hid the unsupported content name and the actionable spelling mismatch.

Impact

Users now see an actionable diagnostic such as:

Item identity 4 contains unsupported DisplayList content “platformView”. Did you mean “platform-view”?

Validation

• swift test — 16 tests passed
• filtered regression test rerun after the fixture rename — passed
• npm test — 4 tests passed
• SwiftWasm release build and Vite production preview build — passed

[github-actions]

Cloudflare Pages preview

Latest PR preview: https://pr-6.display-list-explorer.pages.dev

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