Fix hit testing on stale derived paint lists after node removal

[nicoburns]

Summary

Fixes #624.

paint_children, stacking-context hoisted children, and inline layout data are derived data rebuilt during resolve(). DocumentMutator::remove_and_drop_node updates parent.children but not these lists, so a hit test between the removal and the next resolve iterated stale NodeIds and panicked in Node::hit_inner via self.with(id) (tree().get(id).unwrap()).

hit_inner now does a fallible lookup and skips ids that are no longer live, in all four places that consume derived ids:

// pos/neg z hoisted children + paint_children:
- self.with(id).hit_inner(...)
+ let Some(child) = self.tree().get(id) else { continue };
+ child.hit_inner(...)

// inline text hits: a stale text node id is no longer returned as a HitResult

Regression tests in tests/blitz-tests/tests/hit_test_after_remove_node.rs cover all four paths (plain child, positive/negative z-index hoisted child, inline text); each panicked before the fix.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/30a37a9f486c4e0e8ba8c6cb5197c1fc
Requested by: @nicoburns

WPT results

No changes in test results compared to main.

_{Generated by the WPT workflow.}

[staging-devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR that start with 'DevinAI' or '@devin'.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring