Specify, test, and fix all broken AsComponents<>ComponentBatch interactions from blanket impls

[teh-cmc]

Commit by commit is probably best.

Turns out cascading blanket impls on &dyn T are hard to reason about and don't do what you expect 🤪:

• A single instance (i.e. C: Component) should never be allowed to cast straight to a collection of batches (i.e. AsComponents).
  This is the source of all the bugs. We need to enforce that this goes through a ComponentBatch first, which in turn will be auto-cast to AsComponents.
• Force-casting a simple batch to a collection is now verboten. It still works where it counts though (i.e. rec.log).
  This was extremely dangerous:

  let labels = [
      MyLabel("a".into()),
      MyLabel("b".into()),
      MyLabel("c".into()),
  ];
  
  // This does not do what you think it does (you're logging 3 batches of the
  // same component, with one instance each!!!).
  rec.log("labels", &labels as &dyn crate::AsComponents).unwrap();

[github-actions]

Web viewer built successfully. If applicable, you should also test it:

• I have tested the web viewer

Result	Commit	Link	Manifest
✅	d82fc66	https://rerun.io/viewer/pr/8591	+nightly +main

^{Note: This comment is updated whenever you push a commit.}

[teh-cmc]

@rerun-bot full-check

[github-actions]

Started a full build: https://github.com/rerun-io/rerun/actions/runs/12637853276

[teh-cmc]

@rerun-bot full-check

[github-actions]

Started a full build: https://github.com/rerun-io/rerun/actions/runs/12647234756

[Wumpf]

oof tough! thanks for unraveling this knot!