Sort through examples

Per https://github.com/sveltejs/svelte.dev/pull/294#issuecomment-2401193967:

We will need to audit all the examples closely though. Some don't work, because they make network requests that fail, some are quite dated/strange, some are partially migrated (e.g. the clock example refers to $: in a comment, even though the code now uses $derived), some are showcasing legacy stuff like createEventDispatcher, some are just plain ugly.

A larger question is 'what are these examples for?' Is it weird that many of them are taken directly from the tutorial (while others were taken directly from the tutorial but have since fallen out of date)? If we think duplicating content between tutorial and examples is desirable, then should we formalise that and turn all the app-b directories into examples automatically? (Personally I don't think we should have these duplicates, it's weird. It clutters the search ... and removes the context afforded by the exercise's text and its siblings.) Right now the list is neither concise nor exhaustive; it's a flabby in-between mish-mash. It would be good to develop a clear idea about what belongs here.

Start with the linked pull request comment and inventory the current examples to identify broken, outdated, duplicated, or legacy-focused entries. Done means agreeing what the examples are for and producing a concise, current set that matches that purpose; the issue does not name specific files or tests.